求助一道英语单选题

题目

求助一道英语单选题
_____ is our belief that we all have the responsibility to help create a green world.
A.It
B.This
C.That
D.What
为什么?

答案

that we all have the responsibility to help create a green world是主语,it是形式主语.
it is belief that是……是信仰的意思