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 reasons why birds don"t get lost on long flights ______.

【正确答案】 B

【答案解析】文中第一段说：“从一个地方到另一个地方的长距离飞行中鸟为什么不会迷失方向?科学家们对此问题已困惑多年，现在他们找到了答案。”因此我们可以推断出，对这个问题的答案只是最近才被发现的。因此选项B正确。其他三个选项均与文章内容不符。

单选题 During daylight hours, birds ______.

单选题 By "his outdoor cousins" the author means ______.

【正确答案】 C

【答案解析】文中第三段第四句说：“这只鸽子也像它的outdoor cousins一样朝着一个方向飞。”从第三段中我们得知，这只鸽子是一直生活在笼中且从未在大自然的天空下飞翔过的鸽子。但它同样显示出能以星星作为导航的能力。由此我们可以推断出，与这只鸽子相对比的鸽子一定是在大自然的天空下飞行的鸽子。因此选项C正确。其余三个选项均与文中内容不符。

单选题 The experiment with the dove indicated that ______.

【正确答案】 D

【答案解析】文中第三段第二句、三句说：“然而它显示出与生俱来的用星星作导航的能力。装这只鸽子的笼子是被安放在充满人造星星的天空下面。”结合第二段最后一句，“使用人造星星的实验证明：一些长距离飞行的鸟在夜里能够沿着星星的方向飞”。由此我们可以得出选项D正确。其余三个选项均不符合文章内容。

单选题 In total darkness, doves

【正确答案】 C

【答案解析】文中第四段最后一句说：“但是当天空太黑什么都看不见时，这群鸽子只能无助地在空中打转儿，找不到它们的路。”因此选项C正确。其余三个选项均不符合文章内容。