单选题Directions:This task is the same as Task 1. The five
questions or unfinished statements are numbered from 41 to 45. 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 blares. Not long ago,
experiments showed that birds rely on the sun to guide them during daylight
hours. But what about birds that 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. 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 {{U}}his
outdoor cousins{{/U}}. 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. When the stars are hidden by
clouds, they apparently 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 ______.
A. have been known to scientists for years
B. have only recently been discovered
C. are known by everyone
D. will probably remain a mystery
【正确答案】
B
【答案解析】[解析] 本篇第一段说这个问题多年来使科学家迷惑不解，但随后又说：“Now they're beginning to fill in the blanks.”这里“逐渐开始填补这些空白”意即“逐渐开始对所了解”。
单选题
During daylight hours, birds ______.
A. fly aimlessly
B. rely on landmarks
C. use the sun for guidance
D. are more likely to get lost
【正确答案】
C
【答案解析】[解析] 见第二段第一句：“Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. ”
单选题
By "his outdoor cousins" the author means ______.