---Someone is knocking at the door. Who______it be?---It _______be Vivian. She's in hospital for some time.A.can; can'tB.must; can'tC.can; musn'tD.must; mustn't
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A解析表推测时, can 用于疑问、否定句, must 用于肯定句。注意: 表推测时, 否定句中用can't, 而不是 mustn't。 mustn't是must 的否定形式, 表示禁止。