单项选择题

By the first decade of the 21st century, international commercial air traffic is expected ______beyond today's levels.

A．to have extended
B．to be extending
C．being extended
D．having been extended
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单项选择题
Call your doctor for advice if the ______ persist for more than a few days.
A．responses
B．reflections
C．symptoms
D．syndromes
单项选择题
It is human nature to think back to a Golden Age ______ one's country was strong and respected.
A．unless
B．provided
C．as
D．when
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