单项选择题

The main purpose of this text is to ______.

A．tell students of important schedule changes
B．tell students of new bus and library services
C．show the excellent services for students
D．ask students to renew their library cards

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热门试题

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.

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