is well known to the world is that China has very top technology in space industry．
A. It
B. As
C. What
D. Which
网友回答
A. It
B. As
(答案→)C. What
D. Which
解析：句意：众所周知的事情是中国有太空行业非常高端的技术。第二个is是谓语，前面是主语从句，主语从句缺少主语，用what连接，It可以做形式主语，但是后面第二个is是多余的，As可以引导非限制性定语从句，第二个is应该改成逗号，that可以引导主语从句，但是不能在从句中做成分，所以选C。