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  3. For centuries men dreamed of achieving vertical flight. In 400 AD Chinese children played with a fanlike toy that spun upwards a

For centuries men dreamed of achieving vertical flight. In 400 AD Chinese children played with a fanlike toy that spun upwards a

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问题     For centuries men dreamed of achieving vertical flight. In 400 AD Chinese children played with a fanlike toy that spun upwards and fell back to earth as rotation ceased. Leonardo da Vinci conceived the first mechanical apparatus, called a "Helix", which could carry a man straight up, but this was only a design and was never tested.
    The ancient dream was finally realized in 1940 when a Russian engineer piloted a strange-looking craft of steel tubing with a rotating fan on top. It rose awkwardly and vertically into the air from a standing start, hovered a few feet above the ground, went sideways and backwards, and then settled back to earth. That vehicle was called a helicopter.
    Imaginations were fired. Men dreamed of going to work in their own personal helicopters. People anticipated that vertical flight transports would carry millions of passengers as do the airlines of today. Such fantastic expectations were not fulfilled.     The helicopter has now become an extremely useful machine. It excels in military missions, carrying troops, guns and strategic instruments where other aircraft cannot go. Corporations use them as airborne offices, many metropolitan areas use them in police work, construction and logging companies employ them in various advantageous ways, engineers use them for site selection and surveying, and oil companies use them as the best way to make offshore and remote work stations accessible to crews and supplies. Any urgent mission to a hard to-get-to place is a likely task for a helicopter. Among their other multitude of uses: deliver people across town, fly to and from airports, assist in rescue work, and aid in the search for missing or wanted persons.
Under what conditions are helicopters found to be absolutely essential?
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答案Helicopters are essential for any urgent mission.
解析 从最后一段倒数两句话可看出，在很多紧急情况下而人类又很难到达的地方，直升机的使用是必需的。
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