--- 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
网友回答
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。