题目内容
（请给出正确答案）
When he comes to the shop (late) this afternoon, (then) ask him to remove (all) the cartions (piled) up in the storeroom.
A．late
B．then
C．all
D．piled
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Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly by 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 (鸽子)had 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 an artificial star-filled sky. (76) 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.
(77) But the stars are apparently their principal means of navigation(航行)only. When the stars are hidden by clouds, they seemingly find their way by such landmarks as mountain ranges, coast lines, and river courses. But when it's too dark to see these, the doves circle helplessly, unable to find their way.
The reason why birds don't get lost on long flights ______.
A．have been known to scientists for many years
B．have only recently been discovered
C．are known by us
D．will probably remain a mystery
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