–Where are the students? How could they keep us waiting for so long?
–They started an hour ago and they be here in fifteen minutes, I think.
A. can
B. would
C. should
D. must
网友回答
A. can
B. would
(答案→)C. should
D. must
解析：情态动词辨析。A.能 B. 会 C. 应该 D. 必须。句意：学生们在哪儿：他们怎么能让我们等这么久？——他们一个小时前出发，我认为他们应该在十五分钟内到这儿。表达自己的判断，故选C。