A、The plane flies at high altitudes. B、The plane is filled with air. C、The metal structure of the plane is very strong. D、A smal

Before a new airliner 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 modem airliner must fly at very high altitudes． Air must be pumped into the plane so that the passengers can breathe. The metal structure of the plane has  to be very strong for this reason. When the plane is filled with air, the air presses against the skin of the plane  inside. The pressure on a small window, for example, is like a huge foot that is trying to get out. If a  small part of the plane were to crack, the plane would explode in the sky.. In order to test the structure of the  plane, it 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 in the air. Finally, there is an explosion. Engineers can discover which part of  the plane has cracked. In the second test, the test pilot must find out exactly what happens when all the  engines are shut off at once. He takes the plane up very high. Then he shuts the engines off. The plane begins to fall like a stone. It is 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 can ever carry passengers.