who's the man over there is it mr black ---it __ be him .he's must taller .A.may not B.can't C.will not D.mustn't

who's the man over there is it mr black ---it __ be him .he's must taller .
A.may not B.can't C.will not D.mustn't

B 不可能是（ 否定句中的猜测用CAN‘T ）