－Why didn’t Susan come to see us yesterday as she promised?－.A. She must be busyB. She mig

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A. She must be busy
B. She might have been free
(答案→)C. She must have been busy
D. She can’t be free
解析：考查情态动词表示推测。“情态动词+动词原形”表示对现在情况的推测；“情态动词+have done”表示对过去情况的推测。根据第一句中的didn’t说明本句是对过去情况的推测。排除AD。Might have done本可以做某事，实际上却未坐，这是一个虚拟语气，并非是对过去情况的推测。Must have done sth一定做了某事。句意：—为什么Susn昨天没有像她答应的那样来看我？—她一定是太忙了。故C正确。