Questions 16 to 20 are based on the following passage:
Why don't birds get lost on their long flights from one place to another? Scientists havepuzzled 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 daylighthours. But what about birds that fly by night? Tests with artificial stars have proved that certainnight-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 inbom ability to use the stars for guidance. The bird's cage was placed underan artificial star-filled sky. The bird tried to fly in the same direction as that taken by hisoutdoor cousins. Any change in the position of the artificial stars caused a change in thedirection of his flight.
But the starts are apparently their principal means of navigation (航行) only, when thestars are hidden by clouds, they seemingly find their way by such landmarks as mountainranges, coast lines, and river courses. But when it's too dark to see these, the doves circlehelplessly, unable to find their way.
16.The reason why birds don't get lost on 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正确答案B
