--- is known to us all is thet China has the langest populat

填What,
这里what引导的是一个主语从句,what在主语从句中充当主语
What is known to us all is that China has.
再问： 谢谢，这个是考查主语从句，还是名词性从句，还是两个都有？
再答： 是考查名词性从句，因为主语从句也是属于名词性从句里面的。

同时，想提醒你，要注意以下两种变形：
It is well known to us all that the earth turns around the sun.(这里用it)
As is well know to us all,the earth turns around the sun(这里用as)