They have arrived at lunchtime, but their flight was delayed.
A. will
B. can
C. must
D. should
网友回答
A. will
B. can
C. must
(答案→)D. should
解析：考查虚拟语气。根据后一句their flight was delayed.可知这是对过去情况进行假设。Must have done一定做了某事（表示推测）；should have done本应该做某事实际上却没有做。句意：他们本应该在午饭到，但是他们的航班延误了！