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.isitB.isn'titC.isthisD.isn'tthis
②Tom's never late for class,＿?
A.is heB.isn't heC.has heD.hasn'the
③Nothing is difficult if you put your heart into it,＿?
A.is itB.isn't itC.are theyD.don't you
④You'd better not smoke,＿?
A.did youB.didn't youC.had youD.hadn't you
请帮忙想一下.谢谢.
请说明为什么。谢谢。