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  3. Why don’t birds get lost on their long flights from one place to another? Scientists have puzzled over this question for many ye

Why don’t birds get lost on their long flights from one place to another? Scientists have puzzled over this question for many ye

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问题     Why don’t birds get lost on their long flights from one place to another? Scientists have puzzled over this question for many years. Now they’re beginning to fill in the blanks.
    Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly by night? Tests with artificial stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.
    A dove (鸽子) had spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird’s cage was placed under an artificial star-filled sky. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the artificial stars caused a change in the direction of his flight.
    But the stars are apparently their principal means of navigation (航行) only. When the stars are hidden by clouds, they seemingly find their way by such landmarks as mountain ranges, coast lines, and river courses. But when it’s too dark to see these, the doves circle helplessly, unable to find their way.
The reasons why birds don’t get lost on long flights______.
选项 A、have been known to scientists for years
B、have only recently been discovered
C、are known by everyone
D、will probably remain a mystery
答案B
解析 文中第一段说：“从一个地方到另一个地方的长距离飞行中鸟为什么不会迷失方向?科学家们对此问题已困惑多年，现在他们找到了答案。”因此我们可以推断出，对这个问题的答案只是最近才被发现的。因此选项B正确。其他三个选项均与文章内容不符。
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