my mother thinks ___ important to clean the birdcage every day

题目

my mother thinks ___ important to clean the birdcage every day
a) that
b) this
c) it
d) /
请问选什么?为什么不能用 it's

答案

C
因为it是形式主语,实际主语是clean the birdcage every day
高一要学的.
记住这个用法