【题目】I noticed that he was wearing shoes that did not match. He too hard to notice.
A. might be working B. can have worked
C. must have been working D. should have been worked
在线课程【答案】C
【解析】考查情态动词完成式表示推测。句意：我注意到他穿着不相配的鞋子。他一定是在一直工作很难注意到。must have been doing sth.一定是在做某事，表示对过去事情有把握的肯定推测，故选C。