Other scientists have proposed to use the network for tracking ocean currents and measuring changes in ocean and global temperatures.
The speed of sound in water is roughly one mile a second slower than through land but faster than through air. What is most important, different layers of ocean water can act as channels for sounds, focusing them in the same way a stethoscope does when it carries faint noises from a patient s chest to a doctor s ear. This focusing is the main reason that even relatively weak sounds in the ocean, especially low-frequency ones, can often travel thousands of miles.
31. 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
32. 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
33. The deep-sea listening system makes use of ________.
A) the sophisticated technology of focusing sounds under water
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