Is this red bag yours—No, it isn’t. is that black one.

Have you seen Jack recently? I wonder ______ with his classmates.

I find this Math problem very ___ _. I believe the students can work it out .

(2013·四川，9)Nowadays people are more concerned about the environment________they live.

It was ______ the second-hand car that themechanic spent the whole afternoon in the factory