Before a new plane goes into service, every part of it is tested again and again. But there are two tests that are more important than all the others.
In the first test, a modern airliner must fly at very high altitudes. Air must be pushed into the airliner so that the passengers can breathe. The metal structure of the plane has to be very strong for this reason. When the airliner is filled with air, the air presses against the skin of the plane inside. The pressure on a small window, for instance, is like a huge, giant foot that is trying to get out. If a small part of the plane were to crack, the plane would explode in the sky. To test the structure of the plane, the plane is lowered into a huge tank of water. Then it is filled with air. The pressure inside the plane is greater than it ever will be when it is high in the air. Finally, there is an explosion. Engineers can discover which part of the plane has broken.
In the second test, the test pilot must find ou
A. Because without air the plane can’t fly.
B. Because the plane needs air for its fuel.
C. Because the passengers’ lives depend on air.
D. Because the passengers have paid for it.
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