问题补充：
As a graducate in computer science, he to work out the problem.
A. wouldn have failed
B. can have failed
C. mustn’t have failed
D. might not have failed
答案：
A. wouldn have failed
(答案→)B. can have failed
C. mustn’t have failed
D. might not have failed
解析：can’t/couldn have done表示对过去发生的事情的否定推测。
