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  3. For centuries men dreamed of achieving vertical flight. In 400 A.D. Chinese children played with a fan-like toy that spun upward

For centuries men dreamed of achieving vertical flight. In 400 A.D. Chinese children played with a fan-like toy that spun upward

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问题     For centuries men dreamed of achieving vertical flight. In 400 A.D. Chinese children played with a fan-like toy that spun upwards and fell back to earth as rotation ceased. Leonardo da Vinci con ceive the first mechanical【B1】______,called a "Helix," which could carry man straight up, but was only a design and was never tested.
    The ancient-dream was finally realized in 1940 when a Russian engineer piloted a strange look ing craft of steel tubing with a rotating fan【B2】______.It rose awkwardly and verti cally into the air from a standing start,hovered a few feet above the ground,went【B3】______and backwards, and then settled back to earth. The vehicle was called a helicopter.
    Imaginations were fired. Men dreamed of going to work in their own personal helicopters. Peo ple anticipated that vertical flight transports would carry millions of passengers as do the airliners of today. Such fantastic expectations were not【B4】______.
    The helicopter has now become an extremely useful machine. It【B5】______in military missions, carrying troops, guns and【B6】______ instruments where other air craft cannot go. Corporations use them as airborne offices, many metropolitan areas use them in po lice work, construction and logging companies employ them in various【B7】______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【B8】______crews and supplies. Any urgent mission toa hard-to-get-to placeisa likely task for a helicopter.Among their other【B9】______of use:deliver people across town,fly to and from airports,【B10】______ rescue work, and aid in the search for missing or wanted persons.
【B2】
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