【摘要】
Passage Two
As a result of pollution, Lake Erie, on the border of the U. S. A. and Canada, is now without any living things.
Pollution in water is not simply a matter of “poisons” killing large numbers of fish overnight. Very often the effects of pollution are not noticed for many months or years. The first organisms to be affected are plants or plankton---the food of fish, birds, and other creatures. When their source of food disappears, the fish and birds die too. In this way, a whole food chain can be wiped out, and it is not until dead fish and water birds are seen at the river’s edge or on the seashore that people realize what is happening.
Where do the substances that pollute the water come from? There are two main sources, sewage and industrial waste. As more detergents are used in the home, so more of it is eventually put into our rivers, lakes and seas. Detergents harm water birds by dissolving the natural substances that keep their feathers waterproof. Sewage itself, if it is not properly treated, makes the water dirty and prevents all forms of life in rivers and the sea from receiving the oxygen they need. Industrial waste is even more harmful since there are many highly poisonous substances in it, such as copper and lead.
So, if we want to stop this pollution, the answer is simple: sewage and industrial waste must be made clean before flowing into the water. It may already be too late to save some rivers and lakes, but others can still be saved if the correct action is taken at once.
Questions 29 to 31 are based on the passage you have just heard.
29. What are the main sources of water pollution?
30. Which of the following is harmful according to the passage?
31. What correct action should be taken to stop water pollution?