They have arrived at lunch time but the flight was delayed.A. shouldB. canC. mustD. will

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(答案→)A. should
B. can
C. must
D. will
解析：考察情态动词 should have done本应当干某事 can have done可能做了某事 must have done一定做了某事 will have done 将要做好
句意为，他们本应该在午饭的时候到达，但是航班被推迟了。