is known to everybody that the moon travels round the earth once every month.A. It B. AsC

问题补充：

is known to everybody that the moon travels round the earth once every month.

A. It

B. As

C. That

D. What

答案：

(答案→)A. It

B. As

C. That

D. What

解析：it 为形式主语，真正的主语是后面的 that 从句。