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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 conceive 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 looking craft of steel tubing with a rotating fan【B2】______. It rose awkwardly and vertically 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. People 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 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【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 to a hard-to-get-to place is a 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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答案on top
解析 on top是一个固定表达，表示“在上面”，在这里作方位状语。
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