题干

 如图所示电路,电源电压保持不变.闭合开关S,当滑动变阻器的滑片P从a端滑到b端的过程中,R1、R2的U﹣I关系图象如图所示.则下列判断正确的是(  )

①图线甲是电阻R1的“U﹣I”关系图象

②电源电压为12V

③电阻R1的最小电功率为2.7W

④滑动变阻器R2的最大阻值为30Ω


A:只有①③正确

B:只有①④正确

C:只有②④正确

D:只有②③正确

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答案(点此获取答案解析)

C

同类题3

阅读下面短文,从短文后各题所给四个选项(A、B、C和D)中,选出可以填入空白处的最佳选项,并在答题卡上将该项涂黑

       On our first anniversary, Jack bought a bottle of Casaque, an expensive cologne(古龙香水)for me. "We agreed to save the money for your postgraduate education", I said somewhat half-heartedly as I took it out from a beautifully-wrapped packet. "I am unable not to give something 1 to the world's most beautiful bride, honey!" I held it to my2, and smelled the most pleasant flavor I could ever3. Looking down at my4blue jeans and ugly, ragged tennis shoes, I wondered if I could ever be worthy of it. But I would 5. I used only a little of it every time 6Jack graduated and our life was financially easier. It's a 7the foolish love of a young husband.

       Then one day three-year-old Jim was sitting on the floor8the empty cologne bottle. His lips wet, he was making a(n) 9face. He drank my10cologne! "Jack, come quick," I cried, "can cologne11my baby? Jim drank the rest of it!" "It's mostly just alcohol, honey," Jack12me. Little Jim was fine, and my concern for my child was13. I now felt 14to see the empty cologne bottle.

      A few weeks later, Jack bought another bottle of my beloved cologne. I 15it as much as I had done to the earlier one. 16, it became three-quarters empty. I thought it best to buy another 17. But the shop assistant told me that there wasn't even a maker of that brand any more.

    Now I only use my special cologne on very important occasions, because there will be18to replace it when these last few precious19are gone. But from the bottom of my heart it's not three-quarters empty but still one-quarter full. Perhaps if I'm very careful, I can 20using it for the rest of my life.