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.

      One of the most urgent environmental problems in the world today is the shortage of clean water.Having clean drinking water is a basic human right. However, acid rain, industrial pollution and garbage have made many sources of water undrinkable. Lakes, reservoirs and even entire seas have become vast pools of poison. Lake Baikal in Russia is one of the largest lakes in the world. It contains a rich variety of animals and plants, including 1, 300 rare species that do not exist anywhere else in the world. Yet, they are being destroyed by the massive volumes of industrial effluent which pour into the lake every day. Even though laws have been made, the government do not have the power to enforce them. Most industries simply ignore the regulations. The Mediterranean Sea occupies 1% of the world's water surface, but it is the dumping ground for 50% of all marine pollution. Almost 16 countries regularly throw industrial wastes a few miles off shore.

       Water is free to everyone. A few years ago people thought that the supply of clean water in the world was limitless. Today, many water supplies have been ruined by pollution and sewage. Clean water is now scarce, and we are at last beginning to respect this precious source. We should do something now.

According to the writer, one environmental problem in the world today is ________