With a modal verb:He 1 have completed his work; otherwise, he wouldn't be enjoying himself by the seaside.
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【正确答案】 1、正确答案：must    
【答案解析】解析：must have done表示对过去的肯定推测，意为“肯定做过某事”，根据题干可知，句子所要表达的意思是“他肯定完成了工作，否则他不会在海边享乐”。答案为must。