单项选择题

The purpose of the "special instruments" mentioned in the third paragraph_________.

A．magnify the image of the sun
B．block out the sun's intense light
C．measure the amount of energy emitted by the sun
D．photograph the sun
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单项选择题
Besides its main use the noise-killing system can___________.
A．make a car lighter
B．make a car quieter
C．reduce the cost of a silencer
D．improve the performance of a silencer
单项选择题
Which of the following gives a general idea of how the noise-killing system works
A．By producing pressure waves going in the opposite direction.
B．By mixing high frequency sound waves with low frequency sound waves.
C．By mixing new sound waves with the noise and sending them out together.
D．By making copies of the unwanted sound waves and letting them out a little later.
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