--- Listen! David is talking happily with his mother in the next room.　--- It ______ be his mot..

--- Listen! David is talking happily with his mother in the next room.　--- It ______ be his mot..

C

考查情景交际和情态动词。“听！大卫在隔壁正在高兴地和妈妈谈话。”“她不可能是他的妈妈，她妈妈作为一名志愿医生，已经去云南了。”情态动词可表推测。Should“应该”；must“一定”；can’t“不可能”；musn’t“”禁止不，表推测。故选C。