— Tom is never late for work. Why is he absent today ?
— Something to him.
A. must happen
B. should have happened
C. could have happened
D. must have happened
网友回答
A. must happen
B. should have happened
C. could have happened
(答案→)D. must have happened
解析：must have done表示对过去事情的肯定推断，意为：过去一定做了某事。A表示对现在或示来事情的肯定推敲；B意为：本应该发生；C意为：可能已经发生，口气不太肯定，由于前一句意为：汤姆上班从未迟到过，而判断他今天缺席一定是因为出了什么事了。