21.

複選題30. (AB) The sound is focused, and then beamed out in front of the dolphin. (AC) Sound production is also influenced by group types, habitats, and behaviour. (AD) One of the keys to dolphin echolocation is water’s superb conduction of sound. (AE) Many doubt, however, that dolphins have a formal language akin to that of humans. (BC) In the mid-1980s, the U.S. Navy began training dolphins to search for mines using their echolocation. (BD) Dolphins detect sonar to identify obstacles and map their surroundings when navigating through the ocean. (BE) Experiments conducted in the mid-1990s with a bottlenose dolphin named BJ demonstrated this sensitive ability. (CD) Although dolphins do not possess vocal cords, they still “speak” using sounds such as whistles, squeaks, and trills.

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