Question

where is mary she can be in the library

--- Where is Mary?--- She ____ in the library.
A.should be B.must be
C.can be D.must have been
为什么D不对?

Answer 1

must have been 的意思是,Mary到现在肯定还在呆在图书馆
然而问句问的是一个时刻的状况,问Mary在哪里.
你回答 “到现在还在”就不对了,它只是问你这个时刻Mary在哪里而已.用be是对的