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-个非限制性定语从句，which 指 He is always really rude.

吉姆总是很粗鲁，这就是为什么人们倾向于避开他。

另外请比较以下两题(注意连词的有无):

虽然他有很多朋友，谁也不能借给他钱。

(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。