问题补充：
is known to everybody, Taiwan is a part of China. We must unify it.
A. It
B. As
C. Which
D. What
答案：
A. It
(答案→)B. As
C. Which
D. What
解析：先行词是Taiwan is a part of China整个主句，一般由as或which引导定语从句，但which引导的定语从句只能放在主句之后，另外，as有“正如”之意，而which没有，所以用as。顺便说说，原句改为：It is known to everybody that Taiwan is…= What is known to everybody is that Taiwan is…
