问几道关于as在定语从句中用法的题目

问几道关于as在定语从句中用法的题目
1.
the british are not so familiar with different cultures and other ways of doing things,____is ofen the case in other country
a.what b.as c.so d.it
[为什么不能用a.what啊}
2.
he is doing such work___I am doing.
a.as b.which c.that d.what
[为什么不能用d.what]
3.[1]___we all know,China is rich in natural resources.
[2]___is well-known that China is rich in natural resources.
[3]___is well-known ,China is rich in natural resources.
a.which b.as c.it d.that
[[3]中为什么不用which}
麻烦具体说明一下as 在定语从句中的特殊用法