–She looks very happy. She have passed the exam.
--I guess so. It’s not difficult after all.
A. should
B. must
C. could
D. might
网友回答
A. should
(答案→)B. must
C. could
D. might
解析：考查情态动词用法。A应该；B必须，一定；C可能；D也许。Must have done一定做了某事；句意：—她看起来很开心。她一定已经通过了考试。—我认为如此。它一点也不难。根据句意说明是对过去所发生的事情的肯定的推测用must have done.故B正确。