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
Ａ
考察情态动词用法。Must have done一定做了某事；could have done本可以做某事；should have done sth本应该做某事；might have done也许做某事；结合句意可知A正确。句意：—玛丽看起来很难过。她一定考试没有及格。