He have finished his work; Otherwise he wouldnot be enjoying himself by the seaside．A.

问题补充：

He have finished his work; Otherwise, he would．not be enjoying himself by the seaside．

A. should

B. must

C. couldn’t

D. can’t

答案：

A. should

(答案→)B. must

C. couldn’t

D. can’t

解析：考查情态动词表推测的用法。must have done 表示对过去事情肯定的推测，用于肯定句中，句意：他一定完成工作了，否则的话他就不会再海边玩的这么高兴了。所以选B。