题目内容：
阅读文章，回答题：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 __________ 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
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