_____ of the water _____ polluted. We must try to protect our environment.

It is reported that the number of tourists visiting Hong Kong this year is smaller than ________of last year.

David, could you tell me _________ the Olympics take place—Every four years. The 32st Olympics will take place in Tokyo in 2020.

When I got out of the room, I found someone_____________ the tree.

Oh, I had a terrible toothache!—You’d better go to see a doctor and have your tooth out.