it, that还是which
请看这道题：
Jim is always really rude, ______ is why people tend to avoid him.
A. that B. it C. this D. which
此题容易误选 A，因为 that’s why … 是一个常用表达，同学们极易受此影响而选错，当然若此句的空格前有连词 and 或其前的逗号改为分号，则完全可选that（分别构成并列句和两个独立的简单句）。此题的最佳答案是 D，句子后半部分为一个非限制性定语从句，which 指 He is always really rude. 另外请比较以下两题（注意连词的有无）：
(1) He had a lot of friends, none of ______ could lend him any money.
A. whom B. them C. which D. who
(2) He had a lot of friends, but none of ______ could lend him any money.
A. whom B. them C. which D. who
第(1)题选A，none of whom could … 为非限制性定语从句；第(2)题选B，由于句中有并列连词 but，整个句子为并列句。