单选题
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 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. 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 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.
单选题
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
【正确答案】
B
【答案解析】[解析] 见第一段中句子“Scientists have puzzled over this question for many years．Now they’re beginning to fill in the blanks．”。
单选题
During daylight hours, birds ______. A. wheel back and forth B. follow the stars in their flights C. use sun for guidance D. are quite likely to get lost
【正确答案】
C
【答案解析】[解析] 见第二段中句子“...experiments showed that birds rely on the sun to guide them during daylight hours”。
单选题
In the paragraph 3, "his outdoor cousins" means ______. A. other relatives B. the other doves of the same brood (n.窝) C. doves under the natural sky D. other birds in general
【正确答案】
C
【答案解析】[解析] 见第三段句子“The bird tried to fly in the same direction as that taken by his outdoor cousins．该鸟也竭力朝着鸟笼外面的伙伴飞行的方向飞。”可知，这里的外面的同伴指的就是空中飞翔的鸽子。
单选题
The experiment with the dove implied that ______. A. birds should be taught to navigate B. a dove 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] 见第二段中句子“Tests with artificial stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights．”。第三段中用dove做例子证明这句话。
单选题
In total darkness, doves A. use lights to find their way B. don't know which way to fly C. made their return flight by themselves D. wait for the stars to appear
【正确答案】
B
【答案解析】[解析] 见最后一段末句“But when it's too dark to see these，the doves circle helplessly，unable to find their way．”，可知，B为正确选项。