Environment-friendly Olympics requires Beijing to improve its environment, turning it into an ecology-friendly city.
No bread eaten by man is so sweet as ________ earned by his own labor.
It is written clearly in the requirements that all passengers _______ check onto board thirty minutes before the plane takes off.
How disappointing! We have finished _______ 5%
Tom hid himself behind the door, _______ he rushed out to scare his mother.
You can't imagine the hard time he had ______ the horse in.