单项选择题

The noise killing system is not yet popular mainly because

A．it is still being tried out
B．drivers still have their doubts
C．it increases the cost of car production
D．carmakers are not sure if it is necessary
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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
The underlined word event ( Paragraph 1 ) refers to______.
A．landslide
B．machine
C．earthquake
D．flood
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