I asleep because it took me a long time to realize that the telephone was ringing.
A. could have fallen
B. should have fallen
C. must have fallen
D. mustn’t have fallen
网友回答
A. could have fallen
B. should have fallen
(答案→)C. must have fallen
D. mustn’t have fallen
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。