He is so addicted to smoking that ________ seems to be no chance of talking him into quitting.

—        the team finished most of their research work?—Yes, but the rest        rather difficult.

I had _________ entered the classroom when I noticed the headmaster was sitting at the back.

Rather than ______ on a crowded bus, he always prefers　a bicycle．

Never before ________ buying a computer, but now it seems a necessity in my everyday life.