单项选择题

According to this passage, some animals have the girl of______.

A．telling people apart by how they behave
B．typing each other
C．telling good people from bad people
D．recognizing human faces
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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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