--Mary looks very upset. She failed to pass the exam.I guess so. It's very dif

问题补充：

----- Mary looks very upset. She failed to pass the exam.

----- I guess so. Its very difficult after all.

A must have B could have C should have D might have

答案：

解析：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。