单项选择题

The deep-sea listening system makes use of

A．the sophisticated technology of focusing sounds under water
B．the capability of sound to travel at high speed
C．the unique property of layers of ocean water in transmitting sound
D．low-frequency sounds traveling across different layers of water

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单项选择题

The passage is chiefly about ______.

A．an effort to protect an endangered marine species
B．the civilian use of a military detection system
C．the exposure of a U. S. Navy top-secret weapon
D．a new way to look into the behavior of blue whales

单项选择题

Which of the following is true about the U. S. Navy underwater listening network

A．It is now partly accessible to civilian scientists.
B．It has been replaced by a more advanced system.
C．It became useless to the military after the cold war.
D．It is indispensable in protecting endangered species.