单项选择题

Under which of the following conditions are helicopters found to be absolutely essential

A．For extremely high altitude flights.
B．For urgent mission to places inaccessible to other kinds of craft.
C．For rescue work.
D．For high-speed transportation.

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热门试题

单项选择题

How has the use of helicopters developed

A．They are taking the place of high-flying jets.
B．They are used for rescue work only.
C．They have been widely used for various purposes.
D．They are now used for commercial purposes.

单项选择题

Helicopters are set to motion by means of______.

A．one rotating fan in the centre of the aircraft and others at each end
B．a rotating fan underneath for lifting
C．a rotating device on top
D．a combination of rotating devices in front and on top