TOEFL Listening: ETS-TOEFL听力机经 - 4D331OT73WU738MAP$

What does the professor say about the sounds produced by marine animals? A. Their use decreases significantly when an animal is under stress. B. They are generally in the same frequency range as ship noise. C. They are used as a survival strategy by many species. D. Researchers are only beginning to understand their purpose.