2. 外语
  3. Why don’t birds (71) lost on their long flights from one place to another? Scientists have puzzled over this (72) for many

Why don’t birds (71) lost on their long flights from one place to another? Scientists have puzzled over this (72) for many

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问题    Why don’t birds  (71)   lost on their long flights from one place to another? Scientists have puzzled over this  (72)   for many years. Now they are beginning to fill  (73)   the blanks.
   Not long ago, experiments  (74)   that birds rely on the sun to guide them  (75)   daylight, hours. But what about birds  (76)   fly by night? Tests with artificial stars have proved that  (77)   night - flying birds are able to follow the stars in their long - distance  (78)   A dove had spent its lifetime in a cage and had never flown under a  (79)   sky. Yet it showed a(n)  (80)   ability to use the stars for guidance. The bird’s cage was placed under a (n)  (81)   star - filled sky. The bird tried to fly in the position as that  (82)   by his outdoor cousins. Any  (83)   in the position of the make believe stars caused a change  (84)   the direction of his flight.
     (85)   think that doves, when flying in daylight, use the sun for  (86)  . But the stars are  (87)   their principal means of navigation. What do they do when the stars are  (88)   by clouds? Apparently, they find their way by such landmarks as  (89)   ranges, coast lines, and river courses. But when it’s too dark to see  (90)  , the doves circle helplessly, unable to get their bearing(方位).  
选项 A、complete
B、entire
C、artificial
D、some
答案C
解析 和前面的natural相对。
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