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

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单项选择题

The author thinks that the conclusions of any research about people's state of mind are______.

A．surprising
B．confusing
C．illogical
D．questionable

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.