If you really have to leave during class you’d better leave the back door. A. forB. atC.

问题补充：

If you really have to leave during class, you’d better leave the back door.

A. for

B. at

C. by

D. out

答案：

A. for

B. at

(答案→)C. by

D. out

解析：for为了；by借助，通过；across通过（平面）；out出去。根据句意：如果你必须在会议期间离开，你最好从后门出去。选C。