单选题 The problem of pollution Has been caused by several factors, ______ can be kept under control.
A. most of which B. most of them C. most of that D. most of what

【正确答案】 A

【答案解析】[解析] 该题考查“n.+ of +which”引导非限制性定语从句的用法，而that，what不能引导非限制性定语从句。most of which在定语从句中作主语。