是that is why还是which is why

请看这样一道题：

He is always really rude, _________ is why people tend to avoid him.

A. that                        B. it                        C. this                        D. which

【分析】此题容易误选 A，因为 that’s why…是一个常用结构，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（分别构成并列句或两个独立的简单句）。此题的最佳答案是 D，句子后半部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. （选A）

(2) He had a lot of friends, but none of _________ could lend him any money. （选B）

A. whom                    B. them                    C. which                        D. who