Is that the reason _______ you are in favor of the proposal?

Is that the reason _______ you are in favor of the proposal?
A、what
B、of which
C、why
D、for
【正确答案】：C
【题目解析】：the reason 后接 why 引导的从句是定语从句，此时 why 是关系副词，在定语从句中作原因状语。