[例1]Youallthosecalculations!Wehaveacomputertodothatsortofthing.
A.needn’thavedoneB.mustnothavedone
C.shouldn’thavedoneD.can’thavedone
[例2]Itohimbecausehephonedmeshortlyafterwards.
A.oughttohavewrittenB.musthavewritten
C.couldn’thavewrittenD.needn’thavewritten
[例3]Youthoseletters.Whydidn’tyou
A.shouldpostB.musthaveposted
C.shouldhavepostedD.oughttopost