B As a result of pollution, Lake Erie, o

题目：

B

As a result of pollution, Lake Erie, on the borders of the USA 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 because the first organisms（生物体）to be affected are either plants or plankton(浮游生物). But these organisms are the food of fish and birds and other creatures. When this food disappears, the fish and birds die too. In this way a whole food chain can be wiped out, and it’s 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 which pollute the water come from? There are two main sources of sewage（下水道污物）and industrial waste. As more detergent（洗涤剂）is used in the home, so more of it is finally put into our rivers, lakes and seas. Detergents harm water birds, dissolving the natural substances which keep their feathers water-proof (防水). Sewage itself, if 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 as there are many highly poisonous things 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 clear 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.

70. Pollution of water is noticed________.

　　A. when the first organisms are affected　     

B. when a good many fish and birds die

　　C. when poisonous things are poured into water　

D. as soon as the balance of nature is destroyed

71.The living things die because there is no ________in the lake or river.

　　A. water　　　　　            B. fish           　 C. poison　　　　　  D. oxygen

72. Which of the following is harmful according to the passage?

　　A. Organisms              　B. Plants and plankton in the water

　　C. Waste water from cleaning    D. Industrial waste made clear before flowing into the water

73. The way to stop water pollution is to________.

　　A. realize the serious situation clearly

　　B. put oxygen into the river

　　C. make the waste material harmless before flowing into the water

　　D. make special room in the sea for our rubbish