英语：情态动词、反意疑问句?（选择题）

英语：情态动词、反意疑问句?（选择题）
①This is a green jeep,＿?
A.is it B.isn't it C.is this D.isn't this
②Tom's never late for class,＿?
A.is he B.isn't he C.has he D.hasn't he
③Nothing is difficult if you put your heart into it,＿?
A.is it B.isn't it C.are they D.don't you
④You'd better not smoke,＿?
A.did you B.didn't you C.had you D.hadn't you
请帮忙想一下.谢谢.
请说明为什么。谢谢。