| 如图所示，两根相距为l的平行直导轨ab、cd、b、d间连有一固定电阻R，导轨电阻可忽略不计；MN为放在ab和cd上的一导体杆，与ab垂直，其电阻也为R；整个装置处于匀强磁场中，磁感应强度的大小为B，磁场方向垂直于导轨所在平面（指向图中纸面内）；现对MN施力使它沿导轨方向以速度v（如图）做匀速运动；令U表示MN两端电压的大小，则（ ） A．U= B．U= C．U=vBl，流过固定电阻R的感应电流由b到d D．U=vBl，流过固定电阻R的感应电流由d到b  |
答案：A
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| I want you to know about our neighborhood, Tangley Heights, a slum(贫民窟). I didn't go there much after I met Maxine and her disabled son Ronald and their friend, Miss Annie.In 1981 I was 12 years old. I __1__that once you are 12, you are too big for lots of things you used to __2__before, but you are too young for a job.So a whole group of us children were __3__with nothing to do, only standing at the corner to __4__if Maxine would come with popeyed(突眼的)Ronald.Somebody found that Ronald went to a school for mentally __5__kids and that every afternoon a special bus brought him home and that his mother was always there to __6__him off the bus. My mama could always make you feel special.She could see what my teachers could not, but that was a __7__between us.When my mama died, the secret died, too.It was Maxine who __8__me to her neighbor, Miss Annie, when I needed __9__with English.She told me to bring my book to her house every day after school.After a few weeks, Miss Annie said, "Vernon, I have never __10__you for money, but I would like you to help Maxine __11__up her yard." I cleaned up the yard, and then put the __12__in a bag that Ronald held.Ronald stood there __13__ the bag as if it were the most important job on earth.When you first saw Ronald, he looked like a (n) __14__animal, but what I understood was that Ronald's problem was __15__.Being scared made him look strange. | ||||
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vBl
vBl