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
Some birds like to sit on a sheep because ______.
A．they can eat its parasites
B．they enjoy traveling with the sheep
C．they depend on the sheep for existence
D．they find the position most comfortable
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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