is known to us all that the earth is smaller than the sun.A. ItB. WhatC. AsD. which

问题补充：

is known to us all that the earth is smaller than the sun.

A. It

B. What

C. As

D. which

答案：

(答案→)A. It

B. What

C. As

D. which

解析：本题考查名词性从句。It在句中作形式主语，真正的主语为that the earth is smaller than the sun，故选A。