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问题 单项选择题

变压器的基本作用是在交流电路中变电压、变电流、（）、变相位和电气隔离。

A、变磁通

B、变频率

C、变功率

D、变阻抗

答案

参考答案：D

填空题

______÷8=14…6；72是6的______倍；45的3倍是______．
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选择题
— Do you know _________ last night? 
— Sorry, I don't know. But I think he should go to bed early because he will have an English exam this
     afternoon.

A. when he goes to bed
B. when he went to bed
C. when did he go to bed
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