is known to everybody, the moon travels round the earth once every month.
A. It
B. As
C. That
D. Which
网友回答
A. It
(答案→)B. As
C. That
D. Which
解析：考查定语从句。本题考查了非限制性定语从句，as指代后面一句话的内容在句中做主语。It,that不能引导非限制性定语从句;which引导的非限制性定语从句不能放在句首。句意：众所周知，月亮一个月绕地球一圈。故B项正确。