He have completed his work; otherwise, he wouldn’t be enjoying himself by the seaside.A.

He have completed his work; otherwise, he wouldn’t be enjoying himself by the seaside.
A. should
B. must
C. wouldn’t
D. can’t

A. should(答案→)B. mustC. wouldn’tD. can’t解析：考查情态动词。otherwise，he wouldn’t be enjoying himself by the seaside（否则，他就不会在海边游玩了）表明他一定完成了自己的工作。要表示“一定做完了某事”要用must have done sth.的结构。should have done表示“本该做的事实际上未做”；can’t have done则表示“不可能做了某事”。误选其他项的原因是没有理解清楚本题的语境。不能推测出“如果他没完成工作，他是不会去海边的”，言下之意是“他一定已经完成了工作”。句意：他应当完成了他的工作，否则，他就不会在海边游玩了。故B正确。

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