You are not supposed to get off the bus ________ it stops.

The number of the people who _______ cars ______ increasing in China.

One-third of the country ________ covered with trees and the majority of the citizens _______ black people.

 Every possible means ________ to prevent the air pollution，but the sky is still not very clear.

________ seems to be no possibility _______ the shy girl can win the first prize in the contest.