The air in mountain areas is ____ that in big cities.

We should take part in such activities infuture _____ we did yesterday

Can we ________ in the classroom

, have you _____your decayed tooth—Yeah, but it’s terrible . I had _____most unpleasanttime at the dentist’s.

What’s wrong with you, Eric? You look tired.— I ________ to prepare for the final exam last night.