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。