the reason that和the reason why

I don’t know the reason _____you were absent from the meeting, but I am sure that someone will tell me the reason ______ you haven't told me.
A. why; that B. that; why C. because; which D. of which; that
答案是A。
why和that都是引导定语从句，判断的时候可以根据从句中缺不缺主宾语来选择。第一个用why是you were absent from the meeting从句中不缺成分然后选关系副词。第二句中you haven't told me，少了tell的一个双宾语，所以选关系代词that或者which。

我上新东方的gmat时，语法老师说the reason why和doubt whether that都是不太准确的表述，因为reason和why，doubt和whether都有意思相近的部分，连用就会重复了。所以我建议是尽量避免reason why 和doubt whether。

reason 的定语从句中的关系代词。例如，I want to know the reason why he was sent to the hospital. 我想知道他为什么被送到医院.（是问被送到医院的原因 ）
I know the reason that he was ill.我知道原因是他病了（that引导的从句是the reason同位语）

不一样，一个是定语从句，一个是原因状语从句。

the reason why——这就是.的原因,是针对前面的疑问或情况进行阐述,表明最直接,最主要的原因.
the reason that——原因是,表示一部分或全部原因,可以是直接的也可以是间接的次要原因.
why和that都引导了reason的定语从句,但是一般我们从句意上分析,如果表示的是“做什么事的原因”之类,就用why；如果是“事情本身引起的原因”之类,就用that.
不知道你明白了没有,