If you sit in a chair a long time your back may begin to hurt.A. atB. inC. onD. for

问题补充：

If you sit in a chair a long time, your back may begin to hurt.

A. at

B. in

C. on

D. for

答案：

A. at

B. in

C. on

(答案→)D. for

解析：本题考查介词。介词for 用来引导一段时间。句意：如果你坐椅子很长时间，你的背部可能开始受伤。