单项选择题

The passage is mainly about ______.

A．a new regulation for all airlines
B．the defects of electronic devices
C．a possible cause of aircraft crashes
D．effective safety measures for air flight

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单项选择题

We refuse ______ that there are insufficient funds in the great vaults of this nation.

A．to believe
B．to be believed
C．believing
D．have believed

单项选择题

A new technique ______, the yields as a whole increased by 20 percent.

A．working out
B．having been worked out
C．having worked out
D．to have been worked out