I saw him leave the office an hour ago . He at this time . A. mustnt be working B. can

问题补充：

I saw him leave the office an hour ago . He at this time .

A. mustn\t be working

B. can\t be working

C. can\t work

D. may not being working

答案：

A. mustn\t be working

(答案→)B. can\t be working

C. can\t work

D. may not being working

解析：must 表示猜测时只用于肯定句。may 可用于肯定句或否定句，但D项有明显错误。can 用于否定句中则表示“不可能”，本句的意义表示正在进行的动作，选C不合适。