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Many ants forage across the countryside in large numbers and undertake mass migrations; these activities proceed because one ant lays a trail on the ground for the others to follow. As a worker ant returns home after finding a source of food, it marks the route by intermittently touching its stinger to the ground and depositing a tiny amount of trail pheromone – a mixture of chemicals that delivers diverse messages as the context changes. These trails incorporate no directional information and may be followed by other ants in either direction.

Unlike some other messages, such as the one arising from a dead ant, a food trail has to be kept secret from members of other species. It is not surprising then that ant species use a wide variety of compounds as trail pheromones. Ants can be extremely sensitive to these signals. Investigators working with the trail pheromone of the leafcutter ant Atta texana calculated that one milligram of this substance would suffice to lead a column of ants three times around Earth.

The vapor of the evaporating pheromone over the trail guides an ant along the way, and the ant detects this signal with receptors in its antennae. A trail pheromone will evaporate to furnish the highest concentration of vapor right over the trail, in what is called a vapor space. In following the trail, the ant moves to the right and left, oscillating from side to side across the line of the trail itself, bringing first one

and then the other antenna into the vapor space. As the ant moves to the right, its left antenna arrives in the vapor space.

The signal it receives causes it to swing to the left, and the ant then pursues this new course until its right antenna reaches the vapor space. It then swings back to the right, and so weaves back and forth down the trail.

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According to the passage, all of the following statements about plaque are true EXCEPT_________.

. Read the following passage and mark the letter A, B, C, or D on your answer sheet to choose the best answer for the following questions.

In the past, both men and women were expected to be married at quite young ages. Marriages were generally arranged by parents and family, with their children having little chance to say no in the matter. In the past it was not surprising to find that a bride and groom had only just met on the day of their engagement or marriage.

In modern Vietnam, this has changed completely as people choose their own marriage-partners based on love, and in consideration primarily to their own needs and wants. Moreover early marriage is quite illegal.

The traditional Vietnamese wedding is one of the most important of traditional Vietnamese occasions. Regardless of westernization, many of the age-old customs practiced in a traditional Vietnamese wedding continue to be celebrated by both Vietnamese in Vietnam and overseas, often combining both western and eastern elements. Besides the wedding ceremony, there is also an engagement ceremony which takes place usually half a year or so before the wedding. Due to the spiritual nature of the occasion, the date and time of the marriage ceremony are decided in advance by a fortune teller. The traditional Vietnamese wedding consists of an extensive array of ceremonies: the first is the ceremony to ask permission to receive the bride, the second is the procession to receive the bride (along with the ancestor ceremony at her house), the third is to bring the bride to the groom's house for another ancestor ceremony and to welcome her into the family, then the last is a wedding banquet. The number of guests in attendance at these banquets is huge, usually in the hundreds. Several special dishes are served. Guests are expected to bring gifts, often money, which the groom and bride at one point in the banquet will go from table to table collecting.

 According to the passage, in the past, _____.

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.

On the fourth Thursday in November, in houses around the United States, families get together for a feast, or a large meal. Almost all families eat turkey and cranberry sauce for this meal, and have pumpkin pie for dessert. This feast is part of a very special day, the holiday of Thanksgiving.

In 1620 the Pilgrims made a difficult trip across the ocean from England. They landed in what is now Massachusetts. In England the Pilgrims had not been allowed to freely practice their religion. So they went to the New World in search of religious freedom.

The Pilgrims' first winter was very hard. Almost half the group died of cold, hunger and disease. But the Indians of Massachusetts taught the Pilgrims to plant corn, to hunt and to fish. When the next fall came, the Pilgrims had plenty of food. They were thankful to God and the Indians and had a feast to give thanks. They invited the Indians to join them. This was the first Thanksgiving.

Thanksgiving became a national holiday many years later because of the effort of a woman named Sarah Hale. For forty years Sarah Hale wrote to each president and asked for a holiday of Thanksgiving. At last she was successful. In 1863 President Lincoln declared Thanksgiving a holiday.

How much is Thanksgiving today like the Pilgrims’ Thanksgiving? In many ways they are different. For example, historians think that the Pilgrims ate deer, not turkey. The idea of Thanksgiving, though, is very much the same: Thanksgiving is a day on which we celebrate and give thanks.

When did the the Pilgrims make a difficult trip to across the ocean from England?

The underlined phrase “the trend of” in paragraph 1 could be best replaced by which of the following?

According to the first paragraph, which group is most likely to be worried about children’s use of the Internet?

According to the passage, which of the following is true about the traditional Vietnamese

wedding?

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.

The cause of tooth decay is acid, which is produced by bacteria in the mouth. The acid removes minerals from tooth enamel, allowing tooth decay to begin; the saliva in your mouth encourages remineralization and neutralizes the acid. The rate at which bacteria in the mouth produce acid depends on the amount of plaque on the teeth, the composition of the microbial flora, and whether the bacteria of the plaque have been "primed" by frequent exposure to sugar. To keep your teeth healthy, a regular dental hygiene program should be followed.

Removing plaque with a toothbrush and dental floss temporarily reduces the numbers of bacteria in the mouth and thus reduces tooth decay. It also makes the surfaces of the teeth more accessible, enabling saliva to neutralize acid and remineralize lesions. If fluoride is present in drinking water when teeth are forming, some fluoride is incorporated into the enamel of the teeth, making them more resistant to attack by acid. Fluoride toothpaste seems to act in another way, by promoting the remineralization of early carious lesions.

In addition to a regular dental hygiene program, a good way to keep your teeth healthy is to reduce your intake of sweet food. The least cavity-causing way to eat sweets is to have them with meals and not between. The number of times you eat sweets rather than the total amount determines how much harmful acid the bacteria in your saliva produce. But the amount of sweets influences the quality of your saliva. Avoid, if you can, sticky sweets that stay in your mouth a long time. Also try to brush and floss your teeth after eating sugary foods. Even rinsing your mouth with water is effective.

Whenever possible, eat foods with fiber, such as raw carrot sticks, apples, celery sticks, etc., that scrape off plaque, acting as a toothbrush. Cavities can be greatly reduced if these rules are followed when eating sweets.

What does this passage mainly discuss?