He his work, otherwise, he wouldn’t be enjoying himself by the seaside.
A. must have pleted
B. should have pleted
C. ought to plete
D. must plete
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(答案→)A. must have pleted
B. should have pleted
C. ought to plete
D. must plete
解析：句意：他一定是完成了他的工作，否则他不会在海边玩得那么开心。A. must have pleted对过去的肯定推测；B. should have pleted本应该完成，实际上没完成；C. ought to plete应该完成；D. must plete必须完成。表示对现在的推测。故选A。
考点: 考查情态动词表推测的用法。