Which one is ________,the white one or the black one?

–There is thick haze(雾霾)around many cities in China recently. What do you think of it ? ---I think __________ cars we drive, the less pollution the city has.

Which month has ________ days in a year?     February.

The Yellow River is the second _____________river in our country.

Air pollution has become__________ than ever before. We must do something to stop it.