I____ asleep because it took me a long time to realize that the telephone was ringingA.could have fallenB.should have fallenC.must have fallenD.mustn’t have fallen
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C解析考查虚拟语气。句意；我肯定是睡着了，因为用了很长时间我猜意识到电话铃一直在响。“must+have+done”表示对过去事情的肯定推测，译成“一定做过某事”。故选C。