Small dogs generally live longer than big dogs so that small Yorkshire terrier (猎狐狗) next door could be around for a long time.

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问题     Small dogs generally live longer than big dogs so that small Yorkshire terrier (猎狐狗) next door could be around for a long time. But body size isn’t the only factor that【B1】______how long dogs survive. Personality influences life span, too, according to a new study that might help explain how animal【B2】______evolve.
    Study on animals from ants to apes has found that different【B3】______have different personalities. Some are timid, others【B4】______. Biologists have proposed that temperaments (性情) evolved along with life history.【B5】______, violent animals use a lot of energy fast in【B6】______short lives, the thinking goes, whereas calmer animals last longer, saving themselves to【B7】______ later in life. But it’s hard to run evolutionary【B8】______on these personalities in anything longer lived than a fruit fly. So evolutionary biologist Vincent from Canada came up with an idea about dogs. "All these breed differences reflect an experiment on artificial selection," says Vincent.【B9】______—the ability to chase foxes into holes, or herd sheep, or sit attractively on a sofa.
    But evolutionary physiologist Joseph Williams of Ohio State University in Columbus isn’t convinced that 【B10】______. "For me, it still remains to be seen. Dogs are contrary to what you would expect in nature in terms of longevity," he says.【B11】______. On the other hand, a Chihuahua will generally outlast a Saint Bernard.
【B4】

选项

答案aggressive

解析 反义参照题。此空与timid对应,词性相同,词义相对。既然有的“胆怯”,那么其他动物的性格应与“胆怯”正好相反。结合录音,不难填入aggressive。
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