is well known to the world is that China has very top technology in space industryA. ItB

问题补充：

is well known to the world is that China has very top technology in space industry．

A. It

B. As

C. What

D. Which

答案：

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。