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
网友回答
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。