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

问题补充：

－Why didn’t Susan come to see us yesterday as she promised?

－.

A. She must be busy

B. She might have been free

C. She must have been busy

D. She can’t be free

答案：

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。