--- Listen! David is talking happily with his mother in the next room.

　--- It ______ be his mother. She has gone to Yunnan to be a volunteer doctor.

A. should           B. must             C. can’t            D. mustn’t

--- Listen! David is talking happily with his mother in the next room.　

--- It ______ be his mother. She has gone to Yunnan to be a volunteer doctor.

1. A.
   should
2. B.
   must
3. C.
   can’t
4. D.
   mustn’t

--- Listen! David is talking happily with his mother in the next room.
　--- It ______ be his mother. She has gone to Yunnan to be a volunteer doctor.