Why don’t birds get lost on their long migratory (迁移的) flights? Scientists tried their best to find the   1  to this question for many years. The reasons have been discovered only lately.
　　  2  ago experiments showed that birds depend on the sun to guide them   3  . But what about birds that fly mainly by night?  4  with man-made stars have   5  that certain night-flying birds are able to follow the   6  in their long distance flights.
　　One such   7  , a warbler (鸣禽), had spent its lifetime in a   8  and had never flown under a natural sky. Yet is showed its   9  by birth to use the stars for guidance. The bird's cage was put under a man-made star-filled sky at migration   10 . The bird tried to fly   11  the same direction as   12  taken by other birds in the building. Any 13 in the position of the make-believe (虚构的) stars   14  a change in the direction of its flight.
　　Scientists think that warblers,  15 flying in daylight, use the sun for guidance. But stars are clearly their important   16  of navigation(导航). What do they do when the stars are   17  behind the clouds? Clearly, they find their way by such land   18  as mountains, coastlines and river courses. But when it's too   19  to see these, the warblers circle  201  , unable to find out where they were.
1. A. reply           B. answer      C. discovery      D. replies
2. A. Not long        B. Long       C. Centuries       D. Years
3. A. during the night                B. during daylight hours
C. in winter                     D. in the dark
4. A. Examinations    B. Labs       C. Tests           D. In the dark
5. A. been proved     B. found       C. been found      D. proved
6. A. stars           B. moon        C. route          D. sun
7. A. star            B. scientist      C. bird           D. flight
8. A. forest          B. cage         C. nest           D. cave
9. A. strength        B. ability        C. experience     D. practice
10. A. time          B. place         C. way          D. season
11. A. to            B. towards       C. in            D. under
12. A. that          B. which         C. one           D. it
13. A. one          B. change        C. way           D. bird
14. A. caused        B. gave          C. resulted        D. meant
15. A. for           B. when         C. after           D. they are
16. A. ways         B. means         C. objects        D. homes
17. A. shown        B. covered        C. moved         D. hidden
18. A. areas         B. surface         C. marks         D. signs
19. A. far           B. high           C. dark          D. bright
20. A. helplessly     B. quickly         C. easily         D. freely