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  3. The majority of small tooth whales are called dolphins. Most dolphin species are about 6 ft in length, the males averaging 4 to

The majority of small tooth whales are called dolphins. Most dolphin species are about 6 ft in length, the males averaging 4 to

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问题     The majority of small tooth whales are called dolphins. Most dolphin species are about 6 ft in length, the males averaging 4 to 8 in. longer than females. The largest is the killer whale, which can be
ft long and weigh between 8000—10000 lbs. The smallest species is the buffeo, found in the Amazon River. The buffeo rarely grows over 3.9 ft in length and 66 lbs in weight.
    Dolphins once were hunted commercially, especially for the small quantity of valuable oil extracted from palls of the head and used to lubricate delicate watch mechanisms. Cheaper oils have now been found from other sources, and dolphins are no longer hunted for this reason. Many dolphins, however, become accidentally trapped and drowned in tuna nets. Between the years 1959 and 1972, an estimated 4.8 million dolphins died this way. Because of the pressure from animal rights activists and the United States consumers, both domestic and international tuna canners have refused to accept shipments from fishing fleets that do not protect dolphins.
    Dolphins are extremely and almost constantly vocal. They are capable of two kinds of sounds. A specialized mechanism in the nasal passage just below the blow-hole enables them to emit short, pulse-type sounds. These sounds, called clicks, can be produced in such rapid succession as to sound like a buzz or even a duck-like quack. The clicks are used as a form of sonar, in which echoes of sounds from surrounding objects enable the animals to detect obstacles, other dolphins, fish, and even tiny bits of matter in the water. The military use dolphins and this ability to help find water mines.
    Deeper in the respiratory system-presumably in the larynx-dolphins produce another type of sound: a high pitched whistle of squeal, which is capable of rapid pitch changes. The whistles differ from the clicks in being essentially single tones. Apparently dolphins use the whistles to communicate a particular emotional state and thus influence the behaviour of other dolphins. Typically, the squeals denote alarm or sexual excitement.
The sound clicks differ from the squeals in that______.
选项 A、they are different in speeds of spreading
B、they are different in wave length
C、they are different in tones
D、they are different in volumes
答案C
解析
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