Today there are more airplanesmore people than ever before in the skies.
---So you've left the classroom key inside again? ---No.I've got it with me this time,but I have forgotten ______ the door?
The number of smokers in the countryside is usually greater than _______ in cities.
Most of us see a doctor ______ when we get a cold or something.This won't cost us a lot.
May I use your bike? I'm going to see my uncle who _______ this city.
It is playing computer games _______ the boy spends most of his spare time.