Children find_____difficult to sit for hours with little to do while flying.

---- Who knocked at the door?  ---- I’ve no idea. I just pretended nobody was at home, so I didn’t ask who was___. 

I’m moving to the countryside because the air there is much fresher than ________ in the city.

Due to bad weather we have to put ___ off until next week to go camping in the mountains．

Someone is at the door. ________ must be Mr Smith.