单项选择题

Helicopters work with the aid of ______

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

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单项选择题

It is stated in Paragraph 1 that the construction of a new education system ______

A．challenges economists and politicians.
B．takes efforts of generations.
C．demands priority from the government.
D．requires sufficient labor forc

单项选择题

In Paragraph 7, the word inevitably is closest in meaning to ______

A．necessarily
B．surely
C．intentionally
D．unavoidably