It was () back home after the experiment.
A not until midnight did he go
B not until midnight that he went

It was (B) back home after the experiment.
A not until midnight did he go
B not until midnight that he went
选B的人都知道是强调句考点，怎么判断是强调句还是关系副词引导的定语从句，这是题目的根本考查点，如果仅仅知道强调句强调谓语那么等于不会做。
如果midnight后面的内容是修饰的midnight本身，那么就是毫无疑问的定语从句了，用when引导定语从句而不用that（会引起歧义）。判断后句内容是否修饰时间，就要看后面的动作与前面的时间是否有必然的直接联系，给一些例句来帮助理解。你要分清楚时间是作为状语还是仅仅是一个普通的名词。in 2008这种类型的就是状语，如果说it is in 2008，后面就绝对不能用when等，因为这里的时间不是独立的名词了，而是状语的一部分，句子缺少主语，后面用定语从句，主句部分就缺失了。试比较。
It is 2008 when China will hold the 29th OlympicGames.
和It is in 2008 that China will hold the 29th OlympicGames.
前一句整个句子的中心是it's 2008。其他部分都是修饰。后一句的in 2008等一大堆内容不是句子主干，that后面的内容才是。例题见下。
1.问: ——makes her different from the other students?
回答: Honest ,I think .
A: Hwo is it that B:How is that
C:What is it that D:What is that
答案:C
2.It was in this bookshop —— I meet my lod friend yesterday .
A:that B:where
C:in which D:The place
答案:A
3.It was in 1992 —— the young man graduated yesterday.
A:that B.in which
C:in the year D:the year
答案:A
7.It was after he got what he had desired —— he realized it was not so important.
A:that B:when
C:since D:as
答案:A


参考文献：http://blog.sina.com.cn/calendula28