Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.

      The Trump campaign ran on bringing jobs back to American shores, although mechanization has been the biggest reason for manufacturing jobs’ disappearance. Similar losses have led to populist movements in several other countries. But instead of a pro-job growth future, economists across the board predict further losses as AI, robotics, and other technologies continue to be ushered in. What is up for debate is how quickly this is likely to occur.

      Now, an expert at the Wharton School of Business at the University of Pennsylvania is ringing the alarm bells. According to Art Bilger, venture capitalist and board member at the business school, all the developed nations on earth will see job loss rates of up to 47% within the next 25 years, according to a recent Oxford study. “No government is prepared,” The Economist reports. These include blue and white collar jobs. So far, the loss has been restricted to the blue collar variety, particularly in manufacturing.

       To combat “structural unemployment” and the terrible blow, it is bound to deal the American people, Bilger has formed a nonprofit called Working Nation, whose mission it is to warn the public and to help make plans to safeguard them from this worrisome trend. Not only is the entire concept of employment about to change in a dramatic fashion, the trend is irreversible. The venture capitalist called on corporations, academia, government, and nonprofits to cooperate in modernizing our workforce.

       To be clear, mechanization has always cost us jobs. The mechanical loom, for instance, put weavers out of business. But it also created jobs. Mechanics had to keep the machines going, machinists had to make parts for them, and workers had to attend to them, and so on. A lot of times those in one profession could pivot to another. At the beginning of the 20th century, for instance, automobiles were putting blacksmiths out of business. Who needed horseshoes anymore? But they soon became mechanics. And who was better suited?

         Not so with this new trend. Unemployment today is significant in most developed nations and it’s only going to get worse. By 2034, just a few decades, mid-level jobs will be by and large obsolete. So far the benefits have only gone to the ultra-wealthy, the top 1%. This coming technological revolution is set to wipe out what looks to be the entire middle class. Not only will computers be able to perform tasks more cheaply than people, they’ll be more efficient too.

       Accountants, doctors, lawyers, teachers, bureaucrats, and financial analysts beware: your jobs are not safe. According to The Economist, computers will be able to analyze and compare reams of data to make financial decisions or medical ones. There will be less of a chance of fraud or misdiagnosis, and the process will be more efficient. Not only are these folks in trouble, such a trend is likely to freeze salaries for those who remain employed, while income gaps only increase in size. You can imagine what this will do to politics and social stability

According to the passage, which of the following is NOT true about jobs in the future?

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

It  can  be  inferred  that  the  delayed  use  of  exposed  iron  structures  in  traditional  styles  of architecture is best explained by the _____.

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

How did the artistic elite mentioned in the passage react to the buildings at the Paris Exhibition?

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

According to paragraph 3, the architectural significance of the Halle Des Machines was its _____.

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

The word “surpassed” is closest in meaning to _____.

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

The word “it” in paragraph 2 refers to_____.

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

Iron replaced stone and timber in the building of bridges because iron was considered _____.

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

According to the passage, iron was NOT used for beams, columns, and girders prior to the early eighteenth century because _____.

Read the following passage on transport, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.

(1) Iron production was revolutionized in the early eighteenth century when coke was first used instead of charcoal for refining iron ore. Previously the poor quality of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the improvement in refining ore, it  was  now  possible  to  make  cast-iron  beams,  columns,  and  girders.  During  the  nineteenth  century  further advances were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable.

(2)  Iron  was  rapidly  adopted  for  the  construction  of  bridges,  because  its  strength  was  far  greater  than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal  iron  skeleton  for  buildings  had  been  developed  in  industrial  architecture  replacing  traditional  timber beams, but it generally remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potential to span vast areas. As a result, iron became increasingly popular as a structural material for more traditional styles of architecture during the nineteenth century, but it was invariably concealed.

(3)  Significantly,  the  use  of  exposed  iron  occurred  mainly  in  the  new  building  types  spawned  by  the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, where its  practical  advantages  far  outweighed  its  lack  of  status.  Designers  of  the  railroad  stations  of  the  new  age explored  the  potential  of  iron,  covering  huge  areas  with  spans  that  surpassed  the  great  vaults  of  medieval churches and cathedrals. Paxton’s Crystal Palace, designed to house the Great Exhibition of 1851, covered an area of 1.848 feet by  408 feet in prefabricated units of  glass set in iron frames. The Paris  Exhibition of 1889 included both  the widest span and the  greatest height achieved so far with the Halle  Des Machines, spanning 362 feet, and the Eiffel Tower 1,000 feet high. However, these achievements were mocked by the artistic elite of Paris as expensive and ugly follies. Iron, despite its structural advantages, had little aesthetic status. The use of an exposed iron structure in the more traditional styles of architecture was slower to develop.

What does the passage mainly discuss?

Read  the  following  passage  and  mark  the  letter  A,  B,  C,  or  D  on  your  answer  sheet  to  indicate  the  correct answer to each of the questions from 36 to 42. 

DESERTIFICATION

Desertification is the degradation of once-productive land into unproductive or poorly productive land. Since the first great urban-agricultural centers in Mesopotamia nearly 6,000 years ago, human activity has had a destructive impact on soil quality, leading to gradual desertification in virtually every area of the world. 

It is a common misconception that desertification is caused by droughts. Although drought does make land  more  vulnerable,  well-managed  land  can  survive  droughts  and  recover,  even  in  arid  regions.  Another mistaken belief is that the process occurs only along the edges of deserts. In fact, it may take place in any arid or  semiarid  region,  especially  where  poor  land  management  is  practiced.  Most  vulnerable,  however,  are  the transitional  zones  between  deserts  and  arable  land;  wherever  human  activity  leads  to  land  abuse  in  these fragile marginal areas, soil destruction is inevitable. 

[1]  Agriculture  and  overgrazing  are  the  two  major  sources  of  desertification.  [2]  Large-scale  farming requires  extensive  irrigation,  which  ultimately  destroys  lands  by  depleting  its  nutrients  and  leaching  minerals into the topsoil. [3] Grazing is especially destructive to land because, in addition to depleting cover vegetation, herds of grazing mammals also trample the fine organic particles of the topsoil, leading to soil compaction and

erosion. [4] It takes about 500 years for the earth to build up 3 centimeters of topsoil. However, cattle ranching and agriculture can deplete as much as 2 to 3 centimeters of topsoil every 25 years - 60 to 80 times faster than it can be replaced by nature. 

Salination  is  a  type  of  land  degradation  that  involves  an  increase  in  the  salt  content  of  the  soil.  This usually occurs as a result of improper irrigation practices. The greatest Mesopotamian empires- Sumer, Akkad and  Babylon-  were  built  on  the  surplus  of  the  enormously  productive  soil  of  the  ancient  Tigris-  Euphrates alluvial  plain.  After  nearly  a  thousand  years  of  intensive  cultivation,  land  quality  was  in  evident  decline.  In response, around 2800 BC the Sumerians began digging the huge Tigris-Euphrates canal system to irrigate the exhausted soil. A temporary gain in crop yield was achieved in this way, but over-irrigation was to have serious and unforeseen consequences. From as early as 2400 BC we find Sumerian documents referring to salinization as a soil problem. It is believed that the fall of the Akkadian Empire around 2150 BC may have been due to a catastrophic failure in land productivity; the soil was literally turned into salt. Even today, four thousand years later,  vast  tracks  of  salinized  land  between  the  Tigris  and  Euphrates  rivers  still  resemble  rock-hard  fields  of snow.

Soil  erosion  is  another  form  of  desertification.  It  is  a  self-reinforcing  process;  once  the  cycle  of degradation begins, conditions are set for continual deterioration. As the vegetative cover begins to disappear, soil becomes more vulnerable to raindrop impact. Water runs off instead of soaking in to provide moisture for plans. This further diminishes plan cover by leaching away nutrients from the soil. As soil quality declines and runoff  is  increased,  floods  become  more  frequent  and  more  severe.  Flooding  washes  away  topsoil,  the  thin, rich, uppermost layer of the earth’s soil, and leaves finer underlying particles more vulnerable to wind erosion. Topsoil contains the earth’s greatest concentration of organic matter and microorganisms, and is where most of the earth’s land-based biological activity  occurs.  Without this fragile  coat of nutrient-laden material, plan life cannot exist. An extreme case of its erosion is found in the Sahel, a transitional zone between the Sahara Desert and  the  tropical  African  rain  forests;  home  to  some  56  million  people.  Overpopulation  and  overgrazing  have opened  the  hyperarid  land  to  wind  erosion,  which  is  stripping  away  the  protective  margin  of  the  Sahel,  and causing  the  desert  to  grow  at  an  alarming  rate.  Between  1950  and  1975,  the  Sahara  Desert  spread  100 kilometers southward through the Sahel. 

The word “leaching” in paragraph 5 is closet in meaning to _____.

Read  the  following  passage  and  mark  the  letter  A,  B,  C,  or  D  on  your  answer  sheet  to  indicate  the  correct answer to each of the questions from 36 to 42. 

DESERTIFICATION

Desertification is the degradation of once-productive land into unproductive or poorly productive land. Since the first great urban-agricultural centers in Mesopotamia nearly 6,000 years ago, human activity has had a destructive impact on soil quality, leading to gradual desertification in virtually every area of the world. 

It is a common misconception that desertification is caused by droughts. Although drought does make land  more  vulnerable,  well-managed  land  can  survive  droughts  and  recover,  even  in  arid  regions.  Another mistaken belief is that the process occurs only along the edges of deserts. In fact, it may take place in any arid or  semiarid  region,  especially  where  poor  land  management  is  practiced.  Most  vulnerable,  however,  are  the transitional  zones  between  deserts  and  arable  land;  wherever  human  activity  leads  to  land  abuse  in  these fragile marginal areas, soil destruction is inevitable. 

[1]  Agriculture  and  overgrazing  are  the  two  major  sources  of  desertification.  [2]  Large-scale  farming requires  extensive  irrigation,  which  ultimately  destroys  lands  by  depleting  its  nutrients  and  leaching  minerals into the topsoil. [3] Grazing is especially destructive to land because, in addition to depleting cover vegetation, herds of grazing mammals also trample the fine organic particles of the topsoil, leading to soil compaction and

erosion. [4] It takes about 500 years for the earth to build up 3 centimeters of topsoil. However, cattle ranching and agriculture can deplete as much as 2 to 3 centimeters of topsoil every 25 years - 60 to 80 times faster than it can be replaced by nature. 

Salination  is  a  type  of  land  degradation  that  involves  an  increase  in  the  salt  content  of  the  soil.  This usually occurs as a result of improper irrigation practices. The greatest Mesopotamian empires- Sumer, Akkad and  Babylon-  were  built  on  the  surplus  of  the  enormously  productive  soil  of  the  ancient  Tigris-  Euphrates alluvial  plain.  After  nearly  a  thousand  years  of  intensive  cultivation,  land  quality  was  in  evident  decline.  In response, around 2800 BC the Sumerians began digging the huge Tigris-Euphrates canal system to irrigate the exhausted soil. A temporary gain in crop yield was achieved in this way, but over-irrigation was to have serious and unforeseen consequences. From as early as 2400 BC we find Sumerian documents referring to salinization as a soil problem. It is believed that the fall of the Akkadian Empire around 2150 BC may have been due to a catastrophic failure in land productivity; the soil was literally turned into salt. Even today, four thousand years later,  vast  tracks  of  salinized  land  between  the  Tigris  and  Euphrates  rivers  still  resemble  rock-hard  fields  of snow.

Soil  erosion  is  another  form  of  desertification.  It  is  a  self-reinforcing  process;  once  the  cycle  of degradation begins, conditions are set for continual deterioration. As the vegetative cover begins to disappear, soil becomes more vulnerable to raindrop impact. Water runs off instead of soaking in to provide moisture for plans. This further diminishes plan cover by leaching away nutrients from the soil. As soil quality declines and runoff  is  increased,  floods  become  more  frequent  and  more  severe.  Flooding  washes  away  topsoil,  the  thin, rich, uppermost layer of the earth’s soil, and leaves finer underlying particles more vulnerable to wind erosion. Topsoil contains the earth’s greatest concentration of organic matter and microorganisms, and is where most of the earth’s land-based biological activity  occurs.  Without this fragile  coat of nutrient-laden material, plan life cannot exist. An extreme case of its erosion is found in the Sahel, a transitional zone between the Sahara Desert and  the  tropical  African  rain  forests;  home  to  some  56  million  people.  Overpopulation  and  overgrazing  have opened  the  hyperarid  land  to  wind  erosion,  which  is  stripping  away  the  protective  margin  of  the  Sahel,  and causing  the  desert  to  grow  at  an  alarming  rate.  Between  1950  and  1975,  the  Sahara  Desert  spread  100 kilometers southward through the Sahel. 

Paragraph 4 of the passage serves mainly to do which of the following?