______ is well-known, the environment in China is badly in need

______ is well-known, the environment in China is badly in need of improvement.
A、It
B、That
C、As
D、What
【正确答案】：C
【题目解析】：as is well-known, 众所周知，/正像大家都知道的那样，as 引导非限制性定语从句，在从句总做主语，代替逗号后的整个句子