You should take off your shoes before you ___ the new flat.
The air in Beijing is getting much ______ now than a few years ago.
He has two pets. One is a dog, ______ is a cat.
— Will you stay for lunch?— Sorry, I ______. My brother is coming to see me.
I phoned you at six yesterday evening, ______ nobody answered.
— ________ did you last see him?— Two weeks ago.