There is no light in the dormitory

There is no light in the dormitory. They must have gone to the lecture,_____?There is no light in the dormitory. They must have gone to the lecture,_____?A.didn’t theyB.don’t theyC.mustn’t theyD.haven’t theyThe key is DWhy not b

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B.don’t they是一般现在时,D.haven’t they是现在完成时,因为说话时他们去图书馆还没回来.而且They must have gone 中的must表推测,句子主干是they have gone 所以后面反意疑问句应和have gone对应.所以用haven’t they.

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must have done 表推测当起主句表示现在时态时用ＨＡＶＥ．．．当其主句表示过去时态时用ＤＩＤ．．．