look the man at the gate___be our headmaster .he is always standing there every morning

问题描述:

look the man at the gate___be our headmaster .he is always standing there every morning
no,it ____be him .he is holding a meeting in the office now
A.must can't B must mustn't C can't can't Dcan't musn't

can't 看门口的人不可能是我们校长,他每天早上都站在那里
mustn't是不允许的意思,不符合句意