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问题 选择题

如图是各电场的电场强度E随时间t的变化图象，其中能向外发射的电磁波是（　　）

A．

B．

C．

D．

答案

A、图A是稳定的电场，不能产生磁场．故A错误．

B、C、图B与图C是均匀变化的电场，产生恒定不变的磁场，也不能形成电磁波．故B错误，C错误．

D、图D是按正弦函数规律周期性变化的电场，会产生同频率的周期性变化的磁场，能形成电磁场，向外发射电磁波．故D正确．

故选D．

选择题

下列穿过澳大利亚特殊的纬线是[ ]

A．北回归线  

B．南回归线  

C．赤道  

D．南极圈
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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（17）空应选择（）

A.many 

B.a few 

C.a little 

D.fewer
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