7.A【解析】在0~5×10-2s时间内,ab边切割磁感线产生感应电动势,相当于电源,由楞次定律可知感应电流方向沿顺时针方向,则a点的电势高于b点的电势,所以a、b两点间的电势差为U1=3E=3BL=0.15V;在5×10-2~10×10-2s时间内,cd边进入磁场Ⅱ后,ab边和cd边都切割磁感线,都产生感应电动势,线框中感应电流为零,由左手定则可知a点的电势高于b点的电势,所以ab两点间的电势差U2=E=BLv=0.2V;在10×10-2~15×10-2s时间内,ab边穿出磁场I后,只有cd边切割磁感线,由左手定则可知a点的电势高于b点的电势,所以a、b两点间的电势差为≈1n=1Bv=0.05V,A项正确。

第二节One possible version:Dear AliceHow are you doing these days?From your letter, I know that you won first place inthe speech contest of your schooL. Congratulations! Istrongly believe that you have the ability and undoubtedlyyour hard work and determination contribute to your suc-several months since I entered Senior Three. Dur-Sometimes when I meet with some difficulties, I feelenoccupies most of my time, for I know success lies in hardwork. Only by making every effort will I realize my dreamg Peking UniveWelcome to China if it is convenient for you! I promise that I will take you to many places of interest and youcan taste delicious dishes. I know you must like itI'm looking forward to your replyYoursLi H

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