题目内容：

The reason _______ he was late again was that he was caught in a traffic jam in the rush hour.
A.which B.in which C.for which D.of which
为什么是C,不是D?

优质解答

参考柯林斯字典,the reason for something 是固定搭配,如下：
reason
1 reason reasons
词义：
The reason for something is a fact or situation which explains why it happens or what causes it to happen.
所以这题的从句用 for which