Tracking whales is but one example of an exciting new world just opening to civilian scientists after the cold war as the Navy starts to share and partly uncover its global network of underwater listening system built over the decades to track the ships of potential enemies.

　　Earth scientists announced at a news conference recently that they had used the system for closely monitoring a deep-sea volcanic eruption (爆发)for the first time and that they plan similar studies.

　　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 (听诊器)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

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