阅读理解阅读文章，回答41-45题： Not long ago, experiments showed that birds use the sun to guide them during daylight hours. How do birds fly at 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 had spent its lifetime in a cage and had never flown under a natural sky. But it had an inborn ability to use the stars for guidance. The bird' s cage was placed under an artificial 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 artificial stars caused a change in the direction of his flight. But the stars are apparently their main means of navigation. When the stars are hidden by clouds, they apparently find their way by such landmarks as river courses, mountain ranges, coast lines. But when it' s too dark to see these things, the doves circle helplessly, unable to find their way.

单选题 The reasons why birds don' t get lost on their long flights __________

【正确答案】 B

【答案解析】【解析】题干意为“在长途飞行过程中鸟儿不会迷路的原因__________。”从文章第一段最后一句话“Now they’re starting to fillin the blanks．”可以得出答案B。A项“科学家们已经掌握多年了”；C项“每个人都知道”；D项“将有可能继续是一个谜”，都不符合文意。

单选题 During daylight hours, birds __________

【正确答案】 C

【答案解析】【解析】题干意为“在白天，鸟儿__________。”A项“漫无目的地飞行”；B项“依靠路标”；C项“使用太阳导航”：D项“很有可能迷路”。从文章第二段第一句话“Not long a90，experiments showed that birds use the sun to guide them during daylight hours．”可以判断C为正确答案。

单选题 By "his outdoor cousins" the author means __________

【正确答案】 C

【答案解析】【解析】A项“其他实验者”；B项“其他同一窝的鸽子”；C项“在自然天空下飞行的鸽子”；D项“其他鸟类”。根据题意，C为正确答案。

单选题 The experiment with the dove indicates that__________

A、 birds have to be taught to navigate
B、 a bird that has been caged will not fly long distances
C、 some birds cannot fly at night
D、 some birds seem to follow the stars when they fly at night

【正确答案】 D

【答案解析】【解析】题干意为“鸽子的实验表明__________。”选项A从第三段“But it had an inborn ability to use the stars for guidance．”可以排除。解题关键是从第二段的最后一句话“Tests with artificial stars have proved that certain night-flying birds are able to follow the stars in their long—distance flights．”可排除B、C，并推理出D项为正确答案。

单选题 In total darkness, doves __________

【正确答案】 B

【答案解析】【解析】题干意为“在完全黑暗中，鸽子__________。”A项“使用地面标志”；B项“找不到正确方向”；C项“飞回家”；D项“等待星星出现”。从最后一段最后～句“But when it’S too dark to see these things，the doves circle helplessly，unable to find their way．”可以选出正确答案为B。