it 作形式主語（英語專業(yè)的請進(jìn)）,
選哪個(gè)：I think( ) is not good to smoke in public.
A.this B.that C.it D.you
it 作形式主語要具備什么條件,它前面可以是那些詞,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
以上句子可以嗎?在found、think、belive、feel 這些詞之后的it 作主語和賓語都可以嗎?有什么決定它作主語還是賓語?
（我求知心切,請答得具體點(diǎn),）