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问题 选择题

以下实验装置一般不用于分离物质的是

答案

答案：C

题目分析：A．是蒸发溶剂析出晶体。适合分离溶解度送温度的影响变化不大的物质。错误。B．蒸馏是分离沸点差别大的互溶的液体混合物的方法。错误。C．是准确配制一定体积一定浓度的物质的量浓度的溶液时转移溶液的操作，不是分离混合物。正确。D．是洗气，用于除去气体中的杂质的方法。错误。

填空题

一个数的平方等于这个数本身，则这个数为______．
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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.

 

第（9）空应选择（）

A．wanted

B．began

C．decided

D．arranged
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