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边际成本替代率是指在等产量曲线上两种生产要素相互替代的比率。 ()
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Libya has never gone in for weapons of mass destruction programmes, while Iran has done it.
街道办事处管辖的人口一般以1万人为宜。 ()
Before 1990, the US strategic focus is on Europe and the Middle East.
Science seems to be getting closer to answering a very old mystery (奥秘). Homing pigeons (信鸽) can be taken hundreds of miles from their homes. When they are let to go to fly again, they find their way home. Because of this special ability to find home, pigeons have been used as messengers for hundreds of years.
Today people even breed homing pigeons for racing as a sport. The birds are shipped to some chosen place a few hundred miles away. Then all of them are let to go together. The winner is the bird that gets home first. A good racer can make it home from 500 miles away in a single day.
The mystery of the homing pigeon is how it tells direction and how it finds home.
The first part seems to be pretty well answered, and we know of two ways that pigeons tell directions. First, they use the sun. Experiments show that homing pigeons can tell directions by the sun. What happens when the sky is darkly overcast by clouds and no one can see where the sun is？ Then the pigeons still find their way home.
Naturally, people have wondered whether pigeons might have a built-in compass—something that would tell them about the direction of the earth’s magnetic (磁的) field. Many different kinds of experiments were done. Here’s what the scientists decided after they had made experiments many times. When pigeons can see the sun, they use it as their main means(手段)of direction-finding. When they cannot see the sun, they use some special way to sense direction from the earth’s magnetic field.
But how do pigeons know which direction is toward home？ What do they use that we would call a map？ These are other questions to be answered.
Pigeons have been used as messengers because ______.
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