I think ____ necessary for a student to master a foreign language.

题目

I think ____ necessary for a student to master a foreign language.
A.this B.that C.its D.it
正确答案是D.为什么?
这样念起来很不通的感觉,我怎么觉得应该是it's?

答案

it 是形式宾语,其实是主语
其实就是省略了is
I think it （is） necessary for a student to master a foreign language.