A.can have happened | B.must have happened |
C.should have happened | D.must happen |
A.must | B.should | C.may | D.Can |
A.could | B.may | C.can | D.have to |
A.must do | B.could do | C.would have done | D.should have done |
A.can | B.could | C.were able to | D.might |
A.shall | B.may | C.will | D.can |
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