题目内容：

They must have been here the day before yesterday,__________?
A.mustn't they B.didn't they
C.mustn't have they D.had they
为什么有yesterday 就要用didn't they?
A.为什么不可以呢?

优质解答

这里的must表示推测不是一般情态动词的用法,所以要看主句的时态,因为是一般过去时（yesterday）,所以要用助动词didn't