it 作形式主语（英语专业的请进）,
选哪个：I think( ) is not good to smoke in public.
A.this B.that C.it D.you
it 作形式主语要具备什么条件,它前面可以是那些词,例如：
I found it's interesting to play sports.
I think it's interesting to play sports
I belive it's interesting to play sports
I feel it's interesting to play sports
以上句子可以吗?在found、think、belive、feel 这些词之后的it 作主语和宾语都可以吗?有什么决定它作主语还是宾语?
（我求知心切,请答得具体点,）