Question

I saw him leave the office an hour ago . He ____ at this time . A．mustn't be working B．ca.

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

Answer 1

B。

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。