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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 a long

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

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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 a long time. Now they’ re starting to fill in the blanks.
   Not long ago, experiments showed that birds use the sun to guide them during daylight hours. How do birds fly at 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 had spent its lifetime in a cage and had never flown under a natural sky. But it had 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 main means of navigation. When the stars are hidden by clouds, they apparently find their way by such landmarks as river courses, mountain ranges, coast lines. But when it’ s too dark to see these things, the doves circle helplessly, unable to find their way.
The reasons why birds don’t get lost on their 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 从文章第一段最后一句话“Now they’re starting to fill in the blanks．”可以得出答案B。A项“科学家们已经掌握多年了”；C项“每个人都知道”；D项“将有可能继续是一个谜”，都不符合文意。
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