＿＿＿used to be thought that the earth was square. A. HeB. WhatC. ItD. That

问题补充：

＿＿＿used to be thought that the earth was square.

A. He

B. What

C. It

D. That

答案：

A. He

B. What

(答案→)C. It

D. That

解析：it作形式主语，代替真正主语that从句。