第一部分：单项选择—Make sure you have turned off the air conditioners before going out.—_______________.

How often do you go to a concert?－______ever. I’m not interested in concerts at all.

I don’t know _______ / we? / Mr. White comes from?

Pollution is a serious problem. Scientists have _______ some good ways to save it.

By the way, how many of us can attend the meeting? —The number of the students to attend the meeting _____ fifty.