---------Where is Mr Li?--------He in the office. I saw him leave just now.A. must work

问题补充：

---------Where is Mr Li?

----------He in the office. I saw him leave just now.

A. must work

B. can’t be

C. might be

D. must be working

答案：

A. must work

(答案→)B. can’t be

C. might be

D. must be working

解析：must work必须工作；can’t be不可能……；might be可能； must be working肯定正在工作。句意：李先生在哪儿？根据下文，我看见他刚才离开了。可知前文指的是，他不肯能在办公室里面。故选B。