2. 外语
  3. When animals make long journeys across places where there is no food or shelter, such as deserts or oceans, it is very important

When animals make long journeys across places where there is no food or shelter, such as deserts or oceans, it is very important

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问题     When animals make long journeys across places where there is no food or shelter, such as deserts or oceans, it is very important that they should be able to find their way accurately.  
    Birds find their way by the stars at night and by the sun during the day. People thought that this was impossible. Then some scientists made an experiment. They put some migrating birds(候鸟)in cages inside a planetarium(天象图). When the operator turned the artificial sky(人造天空)round, the birds began to fly to the sides of the cage. Every time he moved it, the birds moved to a new position. Scientists discovered that they always flew toward the direction in which they would have flown if the sky had been real. This proved that they could see the stars and respond to them.  
    Many animals, especially birds, have a very precise sense of time, which is called their “internal clock”. In cloudy weather, birds delay setting off on long journeys, but if the cloud lasts for a long time, they must go at last, to complete their migration in time. In such conditions, they are able to steer(引导)by following the magnetic field(磁场)of the earth. We could say that they have their own special compass inside them, which tells the right direction to go.  
    Most migrating animals travel in groups, sometimes in very large numbers. This makes it more likely that they will find the right way, especially when the group has experienced animals which have made the journey before.
When the cloud lasts for a long time. birds are able to ______ .
选项 A、delay setting off on journey
B、guided by the magnetic field of the earth
C、use their “internal clock” to arrive in time
D、tell the right direction by the“internal clock”
答案B
解析 细节题。根据第三段第三句…they are able to steer by following the magnetic field of the earth. 可知. 当多云的天气持续很长时间后, 候鸟能根据地球磁场来辨别方向, 故选B。
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