语法题,I suppose you are not serious,--------?A.do I B.don't I

语法题,
I suppose you are not serious,________?
A.do I B.don't I C.are you D.aren't you
这个什么情况下用don't
________,I should ask them some questions.
A.Should they come to us B.If they come to us
C.Were they come to us D.Had they come to us
Some cities have passed laws that allow coal and oil_____only if their sulfur content is low.
A.burning B.to burn C.being burned D.te be burned
allow sb to do 和allow sb dong 都可以的吧
为什么不选其他的,请大家给我个详尽的解释哈,