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。