四、篇章結構　　Dolphins use sound to detect the size, shape, and speed of objects hundreds of yards away.Fascinating and complex, the dolphin’s natural sonar, called echolocation, is so precise it candetermine the difference between a golf ball and a ping-pong ball based solely on density.Although humans have researched these intelligent marine mammals for decades, much of theiracoustical world remains a mystery.　　  26   Sound waves travel 4.5 times faster in water than they do in the air. Dolphins usethis to their advantage, in ways that would make a superhero envious. Using nasal sacs in theirheads, dolphins send out rapid clicks that pass through their bulbous forehead, or “melon.”   27   The sound wave speeds through the water, bounces off the object under investigation,and is reflected back to the dolphin. Fat-filled cavities in the dolphin’s lower jaw receive thisinformation and auditory nerves conduct it to the middle ear and brain, where an acousticpicture is created. 　　Scientists say that dolphins may also use clicking to communicate with one another.   28   A mother dolphin may whistle to her newborn for days, apparently to imprint a signaturewhistle upon her baby that will enable it to recognize her. It is believed that dolphins usewhistles to identify one another and possibly for other functions, such as communicatingstrategic alerts while hunting in a group, but scientists have yet to crack the code.   29    　　Since the 1960s, American military scientists have studied dolphins, and have trained themto perform such tasks as attaching explosives and eavesdropping devices to enemy ships orsubmarines.   30   In 2003, dolphins were deployed for the first time in a real war situationto probe the seafloor for mines near the Iraqi port of Umm Qasr. 　　Efforts to replicate dolphin echolocation continue to fall short, as humans have yet toachieve the complexity and precision that 50 million years of evolution has bestowed upondolphins. Perhaps if scientists could understand dolphin-speak, they’d have more luck, but fornow the true nature of dolphin communication remains mysterious.(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 usingtheir echolocation.(BD) Dolphins detect sonar to identify obstacles and map their surroundings whennavigating through the ocean.(BE) Experiments conducted in the mid-1990s with a bottlenose dolphin named BJdemonstrated this sensitive ability.(CD)  Although dolphins do not possess vocal cords, they still “speak” using sounds suchas whistles, squeaks, and trills.
【題組】26.