Although ________ after the Civil War, the American blacks still took no important role in the white Americans except as servants and laborers.
---Thank God you’re safe!--- I stepped back, just _______ to avoid the racing car.
Not until I came home last night ________ to bed.
Work hard when young, ______ you will regret.
It’s hot in the shopping mall, because the air-conditioning system ______.
--- It’s the fourth time that heavy smog(雾霾) has hit the city this month.--- It’s high time we _______ measures to fight air pollution.