It's polite______"Thank you".A.say B.to say C.saying D.said 选什么,为什么、?

It's polite______"Thank you".A.say B.to say C.saying D.said 选什么,为什么、?
It's polite______"Thank you".A.say B.to say C.saying D.said 选什么,为什么、?

It's polite______"Thank you".A.say B.to say C.saying D.said 选什么,为什么、?
B
是it's + 形容词 + to do 的结构,再如：
It's important to learn English well.

当然是B 这里的to do结构是实际主语，it是形式主语