______ is well known, light, like heat, is a form of energy.

______ is well known, light, like heat, is a form of energy.
A、It
B、What
C、Just as
D、As
【正确答案】：D
【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,意思是“正如；正像”