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问题 填空题

调节收音机音量的旋钮开关，实际上是一个______，它的原理是改变连入电路中电阻丝的______来改变电阻．

答案

（1）收音机上的“音量”旋钮，能够控制声音的响度．它的实质是一个滑动变阻器．

（2）电阻大小的影响因素：长度、横截面积、材料．在材料和横截面积一定时，改变电阻线连入电路中的长度来改变电阻大小．

故答案为：滑动变阻器；长度．

单项选择题

The first time I remember noticing the crossing guard was when he waved to me as I drove my son to school. He (31) me with a puzzle-all because he waved to me like someone does on seeing a close friend. A big, (32) smile accompanied his wave. For the next few days I tried tO study his face to see if I knew him. I didn’t. Perhaps he had (33) me for someone else. By the time I contented myself with the conclusion that he and I were strangers, we were greeting each other warmly every morning like old friends.

Then one day the (34) was solved. As I approached the school he was standing in the middle of the road (35) his stop sign. I was in live behind four cars. Once the kids had reached the safety of the sidewalk, he lowered his sign and let the cars (36) . To the first he waved and smiled in just the same way he had done to me over the last few days. The kids already had the window down and were happily waving their reply. The second car got the same (37) from the crossing guard, and the driver, a stiff-looking businessman, gave a brief, almost awkward wave back. Each following car of kids on their way to school (38) more heartily.

Every morning I continued to watch the man with interest. So far I haven’t seen anyone (39) to wave back. I find it interesting that one person can make such a difference to so many people’s lives by doing one simple thing like waving and smiling warmly. His (40) armed the start of my day. With a friendly wave and smiling fade he had changed the feelings of the whole neighbourhood.

35（）。

A. drawing back

B. putting on

C. handing in

D. holding out
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多项选择题

核燃料后处理中采用的溶剂萃取法大致包括哪三个步骤（）

A.首端过程

B.溶剂萃取过程

C.中端过程

D.尾端过程
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