请看高考真题：
It must be the reasons _____ Dr. Johnson avoided discussing in his lecture _____ led the academic committee to question his research design.
A. why; that B. which; which
C. which; that D. that; which
表示原因的词reason作先行词的定语从句并不总是why引导。
1) 当先行词reason在定语从句中作宾语、表语、主语等名词性成分时，引导词用关系代词that/which（也可以省略），不用why引导。例：
Rose's teacher didn’t believe her reason that she gave for her being absent from the class.
罗丝的老师不相信她给出的缺课的原因。
从句___she gave for her being absent from the class缺少gave的宾语。完整的句子应该是：She gave the reason for her being absent from the class.先行词reason在从句中充当gave的宾语，所以用that。再看以下例子：
Please give me a reason that/which caused such a big trouble.
请给我一个造成这么大麻烦的原因。
The reason women are drinking is just because they like it.
女人喝酒只是因为她们喜欢喝。
The reason our schools cannot afford better teachers is because people here are poor
我们的学校请不起好老师的原因是因为这里的人穷。
The reason many people fail on diets is that they jump in at the deep end, making a complete change to their eating habits.
很多人节食失败的原因是他们对节食一窍不通，一下子完全改变了饮食习惯。
The reason he didn't have a family was that he didn't want to be tied down.
他没有成家的理由是不想被捆住手脚。
以上4例摘自《柯林斯高阶英汉双解学习词典》
2) 当先行词在定语从句中做原因状语时，用关系副词why做引导词，不用关系代词that/which。例：
That's one of the reasons why at least one child dies and 293 children are injured in a house fire every day.
这就是为什么每天至少有一名儿童在房屋火灾中死亡，293名儿童受伤的原因之一。
I can't think of any reason why he should do that
我想不出来他为什么非要那样做。
Give me one good reason why I should help you.
我为什么要帮你？给我一个充分的理由。
We don't know the reason why she didn't come home.
我们不知道她为什么没回家。
In other words, there's a reason why people like to gamble.
换句话说，这就是人们为什么喜欢赌博的原因。
The reason why the injection needs repeating every year is that the virus changes.
每年需要重新注射的原因是这病毒经常变化。
The reason why he was late was that his car broke down on the way.
他迟到的原因是他的车在路上抛锚了。
This is the reason why Mike didn't finish his homework.
这就是迈克没有完成作业的原因。
One day, mentally exhausted, I wrote down all the reasons why this problem could not be solved.
一天,心力交瘁的我写下了这个问题无法解决的所有原因。
There is every reason why I shouldn't enjoy his company — he's a bore and a fool.
我有充分的理由不愿和他在一起——他又乏味又愚笨。
There's no earthly reason why you shouldn't go.
你完全没有理由不去。
There is no reason why a gifted aircraft designer should also be a capable pilot. In the same way, a brilliant pilot can be a menace behind the wheel of a car.
杰出的飞机设计师没有理由一定也是个出色的飞行员。同样，一个优秀的飞行员或许会是个糟糕的司机。
There's a surplus of physical shopping space for the crowds, which is one reason why stores are downsizing and closing.
实体店缩减和关门的原因之一，就是为人们提供的实体购物空间过剩。
Perhaps the reason why so many universities offer their students so little is they know studying at a top university remains a brilliant investment even if you don't learn anything.
显然，很多大学教授学生很少的东西的原因是他们知道即使没学到任何东西，在上名牌大学仍是绝佳的投资。
There is no earthly reason why they should ever change
他们完全没有必要作出改变。
There is no reason why love shouldn't cross the age barrier
爱没有理由不能跨越年龄的阻碍。
Unless you're ill, there's no reason why you can't get those 15 minutes of walking in daily.
除非你病了，否则你没有理由每天连走15分钟都做不到的。
There's no reason why you can't have a perfectly normal child
你没有理由不能拥有一个完全正常的孩子。
There is no reason why you couldn't live it up once in a while.
偶尔放纵一下也无妨。
There's a reason why women don't read this stuff; it's not funny
女人不读这种东西是有原因的，它并不好笑。
Provided they are fit I see no reason why they shouldn't go on playing for another four or five years.
只要他们身体强健，我看不出他们有什么理由不继续再打四五年比赛。
An adviser said there was no reason why the two countries should remain at odds.
一位顾问说这两个国家不会一直这样争执下去。
There was no obvious reason why this could not be as good a film as the original.
找不出明显的理由来解释为什么这部片子拍得不如原先的好。
通过观察对比可以发现，先行词是the reason的情况下用why引导的从句主干是完整的，不缺主语，宾语或表语。
回到本题上来，我们发现the reasons后面的从句谓语动词discussing缺少宾语，所以排除A选项。再结合第二空填写that构成强调句型，所以本题的正确答案是选项C。
句意：
一定是约翰逊博士在讲座中避免讨论的那些原因导致学术委员会质疑他的研究设计。