Is( )necessary to tell his father everying?it,this,that三个词用哪一个?为什么?又为什么不用另外两个?

问题描述:

Is( )necessary to tell his father everying?
it,this,that三个词用哪一个?为什么?又为什么不用另外两个?

答案为it
在这里做形式主语.
this that没有这种用法.