They ( ) the film at that time.
问题描述：
They ( ) the film at that time.
A.may see B.mustn't see C.must have seen D.can have seen 书上答案是c,为什么不用过去完成式?求帮忙分析.
答
at that time是表示故去时间的状语,不可以用过去完成时态.此题目考查情态动词表推测的用法,选择C.
They (must have seen ) the film at that time.
翻译：那时侯他们一定是看电影.They (must have seen ) the film at that time.是不是现在完成式？不是现在完成时，而是对过去的推测判断。又如：He must have gone to the Great Wall last Sunday。上周日他一定是去长城了。