Let’s go to play football shall we?- Sorry I can’t. I must go over the points the

问题补充：

--- Let’s go to play football, shall we?

--- Sorry, I can’t. I must go over the points the teacher explained I forget them.

A. after

B. when

C. before

D. since

答案：

A. after

B. when

(答案→)C. before

D. since

解析：连词辨析。A.在……之后；B. 当……时 C. 在……之前D. 自从……以来。句意：让我们去踢足球，好吗？——抱歉，我不能。我必须在我忘记他们之前复习老师解释过的要点。故选C。