The supervisor could have prevented the accident,( )it beforehand.
A.正确答案Bhas he known
B.had he known
C.if he knows
D.if he knew
The supervisor could have prevented the accident,( )it beforehand.
The supervisor could have prevented the accident,( )it beforehand.
A.正确答案Bhas he known
B.had he known
C.if he knows
D.if he knew