是that is why還是which is why

請看這樣一道題：

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

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

【分析】此題容易誤選 A，因為 that’s why…是一個常用結構，同學們極易受此影響而選錯，當然若此句的空格前有連詞 and 或其前的逗號改為分號，則完全可選that（分別構成并列句或兩個獨立的簡單句）。此題的最佳答案是 D，句子后半部為一個非限制性定語從句，which 指 He is always really rude. 另外請比較以下兩題（注意連詞的有無）：

(1) He had a lot of friends, none of _________ could lend him any money. （選A）

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

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