阅读理解。 Before a new type of airplane 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. The first is called the 'tank test'. A modern airplane must fly very high in the sky. Air must be pumped into the plane so that the passengers can breathe. The metal structure (结构) 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 (压力) on a small window is like a huge foot that is trying to get out. If a small part of the plane were to fail, the plane would explode in the sky. To test the structure of the plane, the plane is lowered into a huge tank or container of water. Then it is filled with air. The pressure inside the plane is greater than it ever will be when it is high up in the air. Finally, there is an explosion. This does not cause so much damage inside the water tank as it would anywhere else. Engineers can discover which part of the plane has broken. Then that part is made stronger. The most dangerous test happens when the new plane is going through test flights in the air. The test pilot must find out exactly what happens when the engines (发动机) are all shut off at once. 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. 1. By doing the 'tank test', the engineers can find out _____. [     ] A. the power of the airplane engines B. the amount of air in the plane C. the strength of the plane structure D. the pressure inside and outside the plane 2. What will happen to the plane under the 'tank test'? [     ] A. It will be tested by pilots. B. It will be broken. C. It will be made stronger. D. It will be filled with water. 3. According to the text, why are test flights most dangerous? [     ] A. Too much air may get into the plane. B. The plane may explode in the air. C. The pilot may lose control of the plane. D. The engines may be damaged. 4. What might be the most suitable title for the text? [     ] A. How Airplanes Are Made and Tested B. Two Important Tests on Airplanes C. The Importance of Flying Safely D. The Danger of Testing Airplanes