Questions 31 to 35 are based on the following passage:

　　It is hard to track the blue whale, the ocean's largest creature, which has almost been killed off by commercial whaling and is now listed as an endangered species. 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 Navy, they were able to track a particular blue whale for 43 days, monitoring its sounds. This was possible because of the Navy's formerly top-secret system of underwater listening devices spanning 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 monitoring a deep-sea volcanic eruption (爆發(fā)) 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 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 (聽診器) 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

　　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

　　34. It can be inferred from the passage that ________.

　　A) new radio devices should be developed for tracking the endangered blue whales

　　B) blue whales are no longer endangered with the use of the new listening system

　　C) opinions differ as to whether civilian scientists should be allowed to use military technology

　　D) military technology has great potential in civilian use

　　35. Which of the following is true about the U.S. Navy underwater listening network?

　　A) It is now partly accessible to civilian scientists.

　　B) It has been replaced by a more advanced system.

　　C) It became useless to the military after the cold war.

　　D) It is indispensable in protecting endangered species.