单项选择题

A person beating a drum or blowing a trumpet causes vibrations in the air ____ sound waves.

A．calls
B．called
C．is called
D．are called
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单项选择题
The population of many Alaskan cities has ____ doubled in the past three years.
A．larger than
B．as great as
C．more than
D．as many as
单项选择题
If it ____ too much trouble, I'd love a cup of tea.
A．isn't
B．wasn't
C．weren't
D．hadn't been
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