Cancer has become the top killer ________ I think threatens the health of millions of people. A. tha

Cancer has become the top killer ________ I think threatens the health of millions of people.
A. that B. of which C. 不填 D. Both A and C
【答案】A
【解析】
【详解】考查定语从句。句意：我认为癌症已经成为威胁数百万人健康的头号杀手。分析句子可知，I think 为插入语，可暂且不管；这是一个定语从句，且先行词是top killer，从句中缺主语，故可用that或which。结合选项，故选A。