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Read the passage and fill in the blanks with proper words or the proper forms of the given words.

Why don’t birds get lost on their long migratory (迁移的) flights? Scientists tried their best 1. (find) the answer to this question for many years. The reasons 2. (discover) lately.

Not long ago experiments showed that birds depend 3. the sun to guide them during daylight hours. But what about birds 4. fly mainly at night? Tests with man-made stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.

One such bird, a warbler (莺), had spent its lifetime in a cage and had never flown under a(n) 5. (nature) sky. Yet it showed its ability by birth to use the stars for guidance. The bird’s cage 6. (put) under a man-made star-filled sky at migration time. The bird tried to fly in the same direction as the one that 7. (take) by other birds in the building. Any change in the position of the make-believe (虚构的) stars caused a change in the direction of its flight.

Scientists think that warblers, 8. they fly in daylight, use the sun for guidance. But stars are clearly their important means of navigation (导航). What do they do when the stars are hidden behind the clouds? Clearly, they find their way by such landmarks as mountains, coastlines and river courses. But when it’s too dark 9. (see) these, warblers circle 10. (help), unable to find out where they were.

高一英语语法填空中等难度题

少年,再来一题如何?
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