单项选择题

A．even if
B．if only
C．only if
D．as if
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单项选择题
Under what conditions are helicopters found to be absolutely essential
A．For overseas passenger transportation.
B．For extremely high altitude flights.
C．For high-speed transportation.
D．For urgent mission to places inaccessible to other kinds of craft.
单项选择题
What is said about the development of the helicopter
A．Helicopters have only been worked on by man since 1940.
B．Chinese children were the first to achieve flight in helicopters.
C．Helicopters were considered more dangerous than the early airplanes.
D．Some people thought they would become widely used by average individuals.
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