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问题 选择题

有八种物质：①乙酸、②苯、③聚乙烯、④苯酚、⑤2-丁炔、⑥甲醛、⑦邻-二甲苯、⑧环己烯，既能使酸性高锰酸钾溶液褪色又能与溴水反应使之褪色的是（　　）

A．③④⑤⑧

B．④⑤⑦⑧

C．④⑤⑥⑧

D．③④⑤⑦⑧

答案

①乙酸、②苯、③聚乙烯与高锰酸钾、溴水均不反应；⑦邻-二甲苯与高锰酸钾反应，而与溴水不反应；④⑤⑥⑧既能使酸性高锰酸钾溶液褪色又能与溴水反应使之褪色，

故选：C．

选择题

设{an}是任意等比数列，它的前n项和，前2n项和与前3n项和分别为X，Y，Z，则下列等式中恒成立的是（　　）

A．X+Z=2Y

B．Y（Y-X）=Z（Z-X）

C．Y2=XZ

D．Y（Y-X）=X（Z-X）
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单项选择题

It is clear that some chemicals can damage the health of animals and humans. (1) , this is not the only problem that (2) be caused by the careless use of chemicals. Chemicals can also (3) the ecological balance of the environment. If the ecological balance is disturbed, the results can be (4) serious.

The (5) of DDT illustrates the problem. DDT, a chemical which kills insects, at first seemed to be a perfect answer (6) many problems. It would control insects that caused dangerous diseases, (7) insects that caused billions of dollars of damage to crops every year. Governments permitted and even (8) the use of DDT. Farmers in many countries (9) to spray it on their crops. The (10) results were good: Damage to crops (11) down, and profits went up. However, the chemical had effects which the scientists didn’t see in (12) . First, it also killed insects which were the (13) enemies of the harmful insects and which were therefore beneficial to farmers. Second, and perhaps worse, DDT did not kill (14) harmful insect. A few insects had natural resistance to the chemical. They (15) and reproduced in large numbers. In a few years there were large numbers of insects which were not (16) by DDT, and there were (17) insects which could act as natural (18) on these new "super-insects". Finally, it became clear that DDT was not solving the insect problem. In fact, it was making the problem worse. It (19) became necessary to find a second (20) for the effects of the first.

7（）

A.as well as 

B.as well 

C.but 

D.also
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