He fell asleep immediately last night. He ______ very tired.

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问题 He fell asleep immediately last night. He ______ very tired.

选项 A、must be
B、must have been
C、can be
D、could have been

答案B

解析 情态动词。此句是根据结果推测原因,推测做过的事情的原因用虚拟语气的形式来表示,即:must have done表示对过去事情的肯定性推测,注意其否定形式不是在must后面加not,而是用can或could来否定,称之为否定推测。
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