问题
选择题
—I called you at 9 last night，but no one answered. —Sorry，I ________ sleeping.I went to bed very early.
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答案
C
解析 根据答语中的“I went to bed very early”可知，此处为肯定推测，用must，表示问话人打电话时，答话人一定正在睡觉。must have done表示过去肯定做了某事。