Gun control is a subject Americans have argued for a long time..
A. of which
B. with which
C. about which
D. into which
网友回答
A. of which
B. with which
(答案→)C. about which
D. into which
解析：考察定语从句。该句为定语从句，先行词为subject, 在从句中作argue about 的宾语。
Argue about sth 就……话题进行争论
句意为，枪支的控制是一个美国人争论了很长时间的话题。