_______ is well-known to us all is that the 2008 Olympic Games will be held in Beijing, China．

选项
A.As
B.It
C.Which
D.What

为何选d不选a？谢谢

_______ is well-known to us all is that the 2008 Olympic Games will be held in Beijing, China．

答案选：D。本句是一个主语从句。千万不能把他看成是定语从句，区别如下：

例如：As we all know ,China is a contury with a long history.

译文：众所周知，中国是一个历史悠久的国家。

这里的as,就是用做关系代词，修饰整个后面的分句，即，中国是一个历史悠久的国家；

再看这个句子，译文如下：

我们众所周知的是2008年的奥运会将在中国北京举行。

你发现了没有，What is well-known to us all 是在句子当中充当了主语的成分，后面又接了一个表语从句。表语部分是:that the 2008 Olympic Games will be held in Beijing China.
我再拿出个例子你就很清楚了：（主语从句）
What you said just now is so important to us.
译文：刚刚你所说的对我们真是太重要了。