2. 外语
  3. In 1943 Germany’s submarines — the U-boats — were winning the battle of the Atlantic. Suddenly Germany’s U-boat losses doubled i

In 1943 Germany’s submarines — the U-boats — were winning the battle of the Atlantic. Suddenly Germany’s U-boat losses doubled i

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问题     In 1943 Germany’s submarines — the U-boats — were winning the battle of the Atlantic. Suddenly Germany’s U-boat losses doubled in one month. In three months nearly 100 U-boats were sunken, mostly by aircraft. What happened?
    As soon as it became possible to fit radar in the British coastal command aircraft, there was a change. Radar allowed the planes to search large areas of the sea, to find a submarine even at night and in fog, and to attack before the U-boat could go under the water. The Germans began to lose U-boat to these attacks. They guessed that the aircraft were using radar, and they succeeded in finding out the details of radar. German scientists quickly developed an instrument which picked up the radar signal and gave the U-boat warning.
    The new instruments allowed the Germans to come to the surface at night and destroy Allied ships. That is when they began to win the Battle of the Atlantic. Then the British developed a new type of radar set which used a much shorter wavelength. In a few months it was so dangerous for a U-boat to come up that the battle of the Atlantic was almost at an end.
    That is only one example of the many uses of radar in war. What about its uses in peace?
    Radar has made a great difference to the life of a ship’s officer. The radar screen in the wheelhouse shows him every ship that is near him, every piece of land, every buoy. And he can see them clearly at night or in thick fog. He can measure their distance from his own ship, and he knows the speed of the other ships and the direction they are traveling in.
    Radar is a great help to the pilot of an airliner too. Even in thick fog the officers in the control tower at the airport can see his aircraft. They know its exact position — height, distance, direction, speed. They can talk down the pilot to the point where he can actually see the runway. With even more recent systems, using a combination of radar and other instruments on the aircraft and on the ground, the pilot can now land completely blind in perfect safety.
    The airport usually has radar of more than one kind. A very narrow, pencil-like beam is used to discover the exact position of a particular aircraft. The aerial which sends out the signal and receives the reflected signal is pointed straight at the aircraft. A narrow beam of that kind is not suitable for search over a wide area in order to find all aircraft that are near the airport. So a separate rotating aerial is used for that purpose. The original radar combined these two things, as its English name showed: Radio Detection and Ranging.
As indicated by its English name — Radio Detection and Ranging, Radar means to"______".
选项 A、discover the position of an object
B、send out and receive the reflected signal
C、search over a wide area to find an object
D、find something and decide its exact distance
答案D
解析 推理判断题。根据第五、六段的内容，A、C所说的只是雷达的一项功能，属以偏概全，而B不是雷达的功能。D包括了雷达的两大功能。
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