Th problem which the air pollution in cities has become more and more serious.
问题描述：
Th problem which the air pollution in cities has become more and more serious.
这个句子用which 还是只能用of
答
只能用of,用which不正确.
因为用which的话就是用which引导一个定语从句 “which the air pollution in cities”,而这个句子是没有谓语的,所以不正确.