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
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单项选择题
The underlined word they in the last sentence of the first paragraph refers to ______.
A．birds and parasites
B．birds and sheep
C．parasites and sheep
D．sheep, birds and parasites
单项选择题
We learn from the text that corals depend on plants for ______.
A．comfort
B．light
C．food
D．oxygen
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