--- 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.shouldB.mustC.can’tD.mustn’t
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C解析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。