关于反义疑问句的问题

Question

1 比如it is impolite to speak loudly in public isn't it？ 这句话里，为什么后面用isn't it 而不用is it?
2如果动词为could时后面用did ,还是could。比如i didn't think tom could have done it better,这句话里，让你填后面的句子，有四个选项：A did i, B could he C did he D HAVE HE。请问选哪个？为什么
3 few girls liked this clothes,后面用肯定还是否定？为啥？

Answer 1

1,因为不含否定词not
2，B
3，否定 原因同1

Answer 2

前面是否定，后面就是肯定
前面是肯定，后面就是否定