情态动词 问题

could have done- mean the same as it should have been done

all the other three makes no sense if you place it in the blank

should have failed- means that he failed..

other incorrect for the same reason that I've mentioned in the above answer

Most of the students felt rather disappointed at the English party. They say that it ___ better organized.
A. had been
B. had to be
C.must have done
D.could have done
答案是D 请教下理由。。这句话要说明什么意思？----could have done sth.[本可以做某事实际却未能做]

I'm surprised that he ___ in the exam.
A.should fail
B. would have failed
C.may have failed
D. should have failed
答案是D，请教下理由----[should have done sth本应该做某事（但却未做）]