单项选择题

Passage Three

Many strange new means of transport have been developed in our century, the strangest of them being perhaps the hovercraft. In 1953, a former electronics engineer in his fifties, Christopher Cockerell, who had turned to boat-building on the Norfolk Broads, suggested an idea on which be had been working for many years to the British government and industrial circles. It was the idea of supporting a craft on a "pad", or cushion, of low-pressure air, ringed with a curtain of higher pressure air. Ever since, people have had difficulty in deciding whether the craft should be ranged among ships, planes, or land vehicles—for it is something in between a boat and an aircraft. As a ship builder, Cockerell was trying to find a solution to the problem of the wave resistance which wastes a good deal of a surface’s power and limits its speed. His answer was to lift the vessel out of the water by making it ride on a cushion of air, no more than one or two feet thick. This is done by a great number of ring-shaped air jets on the bottom of the craft. It "flies", therefore, but it cannot fly higher—its action depends on the surface, water or ground, over which it rides.

The word "hovercraft" in line two means （）

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