问题
选择题
----- Mary looks very upset. She _______ failed to pass the exam.
----- I guess so. It's very difficult after all.
A must have B could have C should have D might have
答案
答案：A
本题考查情态动词的用法。句意：玛丽看起来很沮丧。她一定没有通过考试。must have done过去一定做过某事。