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,
A. a clock that is built inside the bird
B. a clock that tells correct time
C. an exact sense of direction
D. an accurate sense of time
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