【题目】— It was a relief when the kids were found.
— Yeah. Their parents _____ have been very worried while they were lost.
A.need
B.should
C.must
D.might

【答案】C
【解析】句意：——孩子被找到这是一个极大的安慰。——是的，在他们丢失期间父母一定非常担忧。A. need“需要”； B. should“应该”；C. must “必须”； D. might“或许”。must have done表示对过去事情有把握的肯定推测，“一定是做某事了”，故选C。