A．Because without air the plane can't fly．
B．Because the plane needs air for its refueling．
C．Because the passengers' lives depend on air．
D．Because the passengers have paid for it．
参考答案C
解析:
Before a new airliner goes into service, every part.of it is tested again and again.But there are two teststhat are more important than all the others.In the first test, a modern airliner must fly at very high altitudes.(23) Air must be pumped into the place so that the passengers can breathe.The metal structure of the planehas to be very strong for this reason.When the plane is filled with air, the air presses against the skin of theplane inside.The pressure on a small window, for example, is like a huge foot that is trying to get out.(24) ~a small part of the plane were to crack, the plane would explode in the sky.In order to test the structure of theplane, it is lowered into a huge tank of water.Then it is filled with air.The pressure inside the plane is greaterthan it ever will be when it is in the air.Finally, there is an explosion.Engineers can discover which part ofthe plane has cracked.
(25) In the second test, the test pilot must find exactly what happens when all the engines are shut off atonce. He takes the plane up very high.Then he shuts the engines off, the plane begins to fall like a stone. Itis the pilot's job to find out how he can get control of the plane again.These two tests are examples of how planes are made safe before they ever carry passengers.
Why must air be pumped into the plane?
C.【精解】根据定位句可知，空气必须充进飞机中，这样乘客才可以呼吸，故答案为C.