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It is clear that some chemicals can damage the health of animals and humans. However, this is not the only problem that can be caused by the (36) use of chemicals. Chemicals can also (37) the ecological balance of the environment. If the ecological balance is broken, the (38) can be extremely serious. The history of DDT (39) the problem. DDT, a chemical which kills insects, at first, seemed to be a perfect answer to many problems. It would control insects that caused dangerous diseases, as well as insects that made (40) of dollars of damage to crops every year. Governments (41) and even encouraged the use of DDT. Farmers in many countries began to spray it on their crops. The (42) results were good: Damage to crops went down, and profits went up. However, the chemical had (43) which the scientists did not predict. First, (44) . Second, and perhaps worse, DDT did not kill every harmful insect. (45) . In a few years there were large numbers of insects which were not affected by DDT. Finally, it became clear that DDT was not solving the insect problem. In fact, (46) !
单项选择题
A. rates B. opportunities C. hopes D. chances
A. rates B. opportunities
C. hopes D. chances
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