As we all know computer games with fights have bad effects the teens.A. in B. at C. forD.

问题补充：

As we all know, computer games with fights have bad effects the teens.

A. in

B. at

C. for

D. on

答案：

A. in

B. at

C. for

(答案→)D. on

解析：介词辨析.in在里面;at在一点;for为了;on在一面;句意:众所周知,带有战斗的电脑游戏对于青少年有很不好的影响.have effects on sth，表示对…有影响。故选D