在题图所示电路中，电池均相同，当电键S分别置于a、b两处时，导线MM′与NN′，之间的安培力的大小为fa、fb，判断这两段导线（ ）
A．相互吸引，fa＞fb
B．相互排斥，fa＞fb
C．相互吸引，fa＜fb
D．相互排斥，fa＜fb
当电键S置于a处时电源为一节干电池电流的方向是M′MNN′，电流大小为Ia=
，由于导线MM′与NN′中电流方向相反故两段导线相互排斥；E R
当电键S置于b处时电源为两节干电池，电流的方向仍是M′MNN′，由于导线MM′与NN′中电流方向相反故两段导线相互排斥．
又由于电路中灯泡电阻不变，此时电路中电流大小为Ib=
；2E R
显然Ib＞Ia，MM′在NN′处的磁感应强度Ba＜Bb，
应用安培力公式F=BIL可知fa＜fb．
综上所述这两段导线相互排斥，fa＜fb．
故D正确，
故选D．
A trip to the bookstore with my granddaughter is always a big event. She __36__ in it for hours, touching the books, running her hands over the covers, looking at a few ___37___ here and looking at a few pages there. I never ___38__ the full importance my five-year-old Grand Angel placed on these trips, until the day I suggested she pick the book she liked best, __39____ we could go home for dinner. Her response（答复） was __40___. “But Grammy, I ___41___ them all the best. All the covers are be-yoo-tiful and all the pictures are bee-yoo-tiful and they’re all __42___. Just like people. Remember when you told me about people?” I most certainly __43___ when I told her about people. I had explained that we are all __44___ in our own different way. Each of us has a story to ___45___ and no two stories are similar. We learn by ____46__ our stories and listening to the stories of others. Difference is the thing that makes each and every one of us __47___. I had no idea that she’d connected the lesson with ___48___. Actually, no one book was ___49___ than any other book. They were equally beautiful and equally special(特别的). With dinner still ___50___ and our stomachs beginning to make noise, ___51___ time was finally at hand. But how to choose? I suddenly ___52___ the right question when I asked, “Which book wants to go home the most with you today?” After a short moment of ___53___, her eyes lit up. She ran to a specific book and ___54___ it from its place on the shelf. The main __55____ was the picture on the cover, a turtle with sad eyes. “We need to find out why the turtle is sad,” she said.
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