Each passage is followed by some questions.For each of-|||-them there are four choices marked A.,B.,C.,and D..You should decide which ONE is the best answer.-|||-Then blacken the corresponding letter on the answer card with a pencll.-|||-Passage One-|||-Questions 21 to 25 are based on the following passage:-|||-Air is all around you.You cannot see it,smell it,or taste it,but you can feel the wind blow You-|||-can see the wind move waves on the water,clouds in the sky,and tree branches.Wind is moving air.-|||-Without air you could not breathe.There could be no living plants or animals. Because sound-|||-travel through air,without air there would be silence,the movement of invisible air can support a-|||-large heavy airplane.Air makes up a precious blanket of atmosphere wrapped around the earth-|||-Beyond this blanket lies airless space,men must carry their own air supplies to be able to live and-|||-work when they travel in spaceships through this airless spacc.-|||-Air is a mixture of gases and water vapor.The most important gases in the air are nitrogen and-|||-orygen.About 78 percent of the air is made up of nitrogen and about 21 percent of orygen.The-|||-remaining one percent is mostly argon (盆),plus very tiny amounts of some other gases.Almost all-|||-living things use the oxygen in air.Fire cannot buru without oxygen.-|||-21.You can feel the air when it __-|||-A.is still B.is around you-|||-C.moves D.keeps quiet-|||-22.You_ __ sound if there is no air.-|||-A.can hear B.can`t hear any-|||-C.can make D.are still able to hear-|||-23.All air around the earth is called __-|||-A.atmosphere B.blanket-|||-C.space D.air supplies-|||-24.Living things depend mostly on the __ air.-|||-