正确答案是：
all of them think it a good thing to grow up
句子中的it是形式宾语,代替了后面的不定式to grow up
例如：
People are going to find it difficult to get to work.
He thought it easy to pass the exam.
I think it wrong for people to behave like that.
句子中的it都是形式宾语,后面不加is