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 underwater listening system was originally designed ______.

A．to trace and locate enemy vessels
B．to monitor deep-sea volcanic eruptions
C．to study the movement of ocean currents
D．to replace the global radio communications network

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