Was it because of your smoking _____ made her cough?
A.which B.what C.who D.that
答案是D。
这个句子看起来不像是强调句（去掉it is that读不通，没主语），那么它是什么结构？？？
（It was because of their hard work that they completed the task in seven weeks instead of nine.这句话显然是强调句了。）
这怎么会是强调句的疑问形式啊，去掉it is that根本读不通！Because of又不能作主语！
各位最好详细解释一下。

答：这个句子完全把人搞糊涂了。乍一看这句子，同你一样，我先把它当强调句来分析。可正如你所说，that后面没有主语，所以不可能是强调句。强调句能恢复成不是强调的原句子。可这个句子如果去掉that恢复成原句，根本就不知道哪个是主语。
请看下面两个强调句的例子：
It is i who did it恢复原句是：i did it.
It is because of his smoking that he got the disease of caner恢复原句是：He got the disease of caner because of his smoking。
如果去年because of则成为强调句Was it your smoking _that_ made her cough，强调的是主语your smoking，恢复成原句是
Your smoking made her cough。
后来我把that后面当主语从句，把it当形式主语来分析，结果还是说的通，因为如果那样，that在主语从句中只能是主语，并且省略了一个引导主语从句的that.可当主语的代词that代表什么，根本不清楚，句子意思不通。
最后我把that后面当定语从句来分析。这样一分析，结果发现语法上是对的，意思上也说得通。句子直译是：它正是因为你那使他咳嗽的吸烟吗？如果改为陈述句则是： it was because of your smoking _that_ made her cough.句子的结构是
it是句子的主语（真正的主语，不是形式主语），because of 及其后面是介词结构，作表语。your smoking _that_ made her cough为介词宾语，其中that_ made her cough作修饰your smoking 的定语从句。但存在的另一个问题是如果真是定语从句which也是正确答案。看来原题似乎有点问题。