789.Sorry,Professor Smith.I didn't finish the assigment yesterday.
Oh,you _____ have done it as yesterday was the deadline.
A.must
B.mustn't
C.should
D.shouldn't
790.Some people who don't like talk much are not necessarily shy;they _________ just be quiet people.
A.must
B.may
C.should
D.would
789.答案是C;此处表示和过去事实相反的情况，should+完成时，表示本来应做而未做。
790.答案是B;may此处表示推测；也许。
