A、In the air. B、On the airfield. C、Under water. D、At plane manufacturing factory. A

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. One of them is very strange and the other is very dangerous.
    The first of these is called the "tank test". A modern 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. The pressure on a small window, for example, 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. This is what happened to the first Comets. 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. This doesn’t cause so much damage inside the water tank as it would anywhere else. Engineers can discover which part of the plane cracked. This part is made stronger.
    The most dangerous test happens when the new plane goes through test flights in the air. The test pilot must find out exactly what happens when the engines are shut off suddenly. He takes the plane up very high. Then he shut 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 ever carry passengers.
Questions 22 to 25 are based on the passage you have just heard.
22. What is the purpose of the tank test?
23. Why is the tank test carried under water?
24. Where is the second test carried out?
25. What does this speaker mainly talk about?