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 secondslower 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 patients chest to a doctors 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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