I asleep because it took me a long time to realize that the telephone was ringing.
A. could fall
B. need have fallen
C. must have fallen
D. may fall
网友回答
A. could fall
B. need have fallen
(答案→)C. must have fallen
D. may fall
解析：考查情态动词表示推测的用法。must have done 表示对过去事情肯定的推测，句意：我当时一定是睡着了，因为花了我好长一段时间才意识到电话正在响。故选C。