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问题 填空题

给下列数据填上适当的单位

书桌的高度是0.7______，一角硬币的厚度约是2.3______，一只鸡的质量2______，拿起一只鸡蛋的力0.5______．

答案

（1）书桌高度一般不足1m，故填m；

（2）一角硬币的厚度一定在1cm以内，故填mm；

（3）一只鸡的质量一般3-5斤，故填kg；

（4）10只鸡蛋的质量约500g（一斤），一只鸡蛋则约50g，根据重力与质量关系G=mg可得其重力为0.5N，拿起它的力即等于重力大小，故填N；

故答案为：m；mm；kg；N．

选择题

如图所示，水平放置的光滑平行金属导轨间距为L，导轨上有一质量为m、电阻为r的金属棒ab，导轨的另一端连接电阻R，其他电阻均不计，磁感应强度为B的匀强磁场垂直于导轨平面向下，金属棒ab在一水平恒力F作用下由静止开始向右运动．则在这个过程中           （   ）

A．随着ab运动速度的增大，其加速度将减小

B．外力F对ab做的功等于电路中产生的电能

C．棒克服安培力做的功一定等于电路中产生的内能

D．当ab棒的速度为v时，ab两端的电势差为BLv
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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.

12（）

A.need 

B.turn 

C.advance

D.vain
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