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阅读下列短文，从每题所给的四个选项(A、B、C、D)中，选出最佳选项。
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 proved that birds depend on the sun to show them the way during daylight hours．But what about birds that mainly fly at night? Tests with man-made stars have shown that certain night-flying birds are able to follow the stars on their long-distance flights．
Once such bird-a warbler-had spent its lifetime in a cage and had never flown under a natural sky at night．Yet it showed an inborn ability to use the stars．The bird's cage was put under a man-made star-filled sky．The bird tried to fly in the same direction as its outdoor cousins．Any change in the position of the man-made stars caused a change in the direction of its flight．
Scientists think that warblers, when flying in daylight, use the sun for guidance．But stars are their main dependence of flying at night．What do they do when stars are hidden by clouds? They find their way by such landmarks as mountains, beaches and rivers．But when it's too dark to see these, the warblers circle helplessly, unable to get their directions．
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