It's hard to track the blue whale, which has almost been killed off by commercial whaling. Attaching radio devices to it is difficult and visual sightings are too unreliable to give real insight into its behavior. So biologists were delighted early this year when, with the help of the US Navy, they are able to track a particular blue whale for 43 days recording its sounds. This was possible because of the Navy's former top secret system of underwater listening devices across the oceans. 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 observing a deep-sea volcanic eruption for the first time and that they planned similar studies. Other scientists have proposed to use the network for tracking ocean currents and measuring changes in the ocean and global temperatures. 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. The deep-sea listening system makes use of _ . A) the ability of sound to travel at high speed B) the top-level technology of focusing sounds under water C) the unique characteristic of layers of ocean water in carrying sound D) low-frequency sounds traveling across different layers of water
A:
C