北师大版高一英语周报21期答案

短文改错Dear Mr Smith.Im writing to extend my sincerely thanks to you for your goodwill to me during your visit to London.sIncereOriginally I only wanted to call on you but convey my father's regards on my arrival in London the last week. But youinsisted that I stay in your house and attend to my life as if i were one of your family member. Your kindness made myattendedmemberstrip easier and happy than expected.appletWords fail to describe what grateful I am. I would like you to come A China and allow my family to returning yourretunkindness.Yours truly

12.CD【解析】由左手定则可知,正电子从S点射出磁场,A项错误;由轨迹图可知正、负电子在磁场中做圆周运动的偏转角均为60°,正、负电子运动的周期相同,则在磁场中运动的时间相同,可知正、负电子同时离开磁场,B项错误;由轨迹图可知,正、负电子各自离开磁场时,两速度方向的夹角为120°,C项正确;由轨迹图可知,PQ=PS=R=BP,则△PQS为等边三角形,则Q、S两点间的距离为L,D项正确。C

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