After a knock at the door, the children heard his mother’s voice him.
---Couldn't you leave a message for him?---Yes,but I need something ______I can write a note.
---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.