Why don't birds get lost on their long flights from one place to another？ Scientists have

Not long ago, experiments showed that birds depend on the sun to guide them during day-flight hours. But what about birds that fly by night？ Tests with man-made stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.

A dove 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 a man-made 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 make-believe stars caused a change in the direction of his flight.

Scientists think that doves, flying ill daylight, use the sun for guidance. But the stars are obviously their most important means of navigation (or flying). What do they do when the stars are hidden by clouds？ Obviously, they find their way by such landmarks as mountains ranges, coastlines, and river courses. But when it is too dark to see these, the doves circle helplessly, unable to get their bearings.

The reason why birds don't get lost on long flight ______.

A．has been discovered recently

B．is known by everybody

C．still remains a mystery

D．has been known to scientists for years