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  3. There are two main things that make aircraft engineering difficult: the need to make every component as reliable as possible and

There are two main things that make aircraft engineering difficult: the need to make every component as reliable as possible and

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问题     There are two main things that make aircraft engineering difficult: the need to make every component as reliable as possible and the need to build everything as light as possible. The fact that an aeroplane is up in the air and cannot stop if anything goes wrong makes it perhaps a matter of life or death that its performance is absolutely dependable.
    Given a certain power of engine, and consequently a certain fuel consumption, there is a practical limit to the total weight of aircraft that can be made to fly. Out of that weight as much as possible is wanted for fuel, radio navigational (航行的，航运的) instruments, passenger seats, or freight (货物) room, and, of course, the passengers or freight themselves. So the structure of the aircraft has to be as small and light as safety and efficiency will allow. The designer must calculate the normal load that each part will bear. This specialist is called the "stress man". He takes account of any unusual stress that may be put on the part as a precaution (预防, 预防措施) against errors in manufacture, accidental damage, etc.
    The stress man’s calculations go to the designer of the part, and he must make it as strong as the stress man says is necessary. One or two samples are always tested to prove that they are as strong as the designer intended. Each separate part is tested, then a whole assembly—for example, a complete wing, and finally the whole aeroplane. When a new type of aeroplane is being made, normally only one of the first three made will be flown. Two will be destroyed on the ground in structural tests. The third one will be tested in the air.
The first three aeroplanes of a new type______.
选项 A、are used for testing purposes
B、are later broken up for spare parts
C、do not fly
D、are all destroyed
答案A
解析 本题问新型飞机的前三架用于什么目的。文章第三段最后两句话指出，当一个新型飞机制作完毕，一般前三架中仅试飞一架。其中两架将在地面拆毁，用于结构检测，第三架接受空中检测。因此，本题的正确答案应是A“新型飞机的前三架用于测试目的”。
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