问题详情：
-- 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 D. can’t, mustn’t
【回答】
A
知识点：情态动词
题型：选择题
