This is ____book I borrowed yesterday. Isn’t it ____interesting one?

The duty of Project Hope is to help poor children, isn’t it?–Yes, it has built many schools ____ those children can study happily.[ ]

Our country the sixth population census(人口普查)already.

Remember to introduce your friend Stone to me when he arrives, ______?

This math problem is ______ difficult ______ that one. [ ]