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
求教：这道题的正确答案是不是C和D都可以?我认为D最合适，但高中老师给出的答案是C。还请帮忙解答，谢谢！
解答：
首先，感谢您对我的信任和问题交流：
It must be the reasons which Dr. Johnson avoided discussing in his lecture that led the academic committee to question his research design.
一定是约翰逊博士在讲座中避开讨论一些原因导致学术委员会质疑他的研究设计。
解析：
1. 第一空填which或that均可以。因为其后的定语从句缺少宾语，故填引导词which或that，而且在口语或非正式语体中that可以省略；
你肯定是想知道为什么不填why，因为why引导定语从句时，why在从句中充当原因状语，而这个定语从句明星缺宾语。一般Why= for which.
请看：
The reason why he was late for work is that he missed the bus.
他上班迟到的原因是他错过了公交车。
定语从句he was late for work中缺的是原因状语，所以用引导词why。
That’s the reason why she didn’t attend the party.
那就是他没参加聚会的原因。
The reason why she dropped out of school is that she didn’t want to increase the family’s financial burden.
她辍学的原因是因为她不想给家里增加经济负担。
下面两个例子，其定语从句中缺主语或宾语、表语等成分，需要用which/that来引导和并在从句中充当（所缺的）成分：
The reason that/ which he gave for his absence was illness.
他给出的缺席理由是生病了。（定语从句中缺宾语）
I know the reason that caused her to change her mind.
我知道导致她改变主意的原因。（定语从句中缺主语）
另外，一定要区分开reason后的从句是定语从句还是同位语从句：如果从内容上reason=从句；说明从句就是同位语从句，此时只能用that来引导：
The reason that he got stuck in the traffic is so bad.
那个他堵在路上的理由太烂了。
2. 第二空天that 。这个句子整体是个强调句型：It is ……that;强调主语。
不知道我这样的解答，您是否满意，仅供参考。
