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What do scientists think causes dreams？
A．Sleep.
B．Scientists have not found the reason up to now.
C．REM sleep.
D．It is not mentioned in the article, but scientists have known it.
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更多“What do scientists think causes dreams？ A．Sleep.B．Scientists have not found th”相关的问题
What do we learn about scientists in earlier times？
A．They claimed to know nothing about nature.
B．They believed in results from scientific observation.
C．They paid much attention to the problems they didn't understand.
D．They invented false theories to explain things they didn't understand.
Land is becoming desert little by little because ______.
A．plants can't grow there
B．there is not enough rain
C．people haven't clone what scientists wish them to do
D．scientists know little about the deserts
听力原文： Why don't birds got lost on their long migratory flights？ Scientists have puzzled over this question for many years. Now they're beginning to fill in the blanks.
Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly mainly by night？ Tests with artificial stars have proved conclusively that certain night-flying birds are able to follow the stars in their long-distance flights.
One such bird—a warbler—had spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird's cage was placed under an artificial star-filled sky at migration time. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the make-believe stars caused a change in the direction of his flight.
Scientists think that warblers, when flying in daylight, use the sun for guidance. But the stars are apparently their principal means of navigation. What do they do when the stars are hidden by clouds？ Apparently, they find their way by such landmarks as mountain ranges, coast lines, and rivet courses. But when it's too dark to see these, the warblers circle helplessly, unable to get their bearings.
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A．Birds have to be taught to navigate.
B．A bird that has been caged will not.
C．Some birds cannot fly at night.
D．Some birds seem instinctively to follow the stars when flying at night.
Not long ago, experiments showed that birds depend on the sun to guide them during day-flight hours. But what about birds that fly by night？ Tests with man-made stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.
A dove spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird's cage was placed under a man-made star-filled sky. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the make-believe stars caused a change in the direction of his flight.
Scientists think that doves, flying ill daylight, use the sun for guidance. But the stars are obviously their most important means of navigation (or flying). What do they do when the stars are hidden by clouds？ Obviously, they find their way by such landmarks as mountains ranges, coastlines, and river courses. But when it is too dark to see these, the doves circle helplessly, unable to get their bearings.
The reason why birds don't get lost on long flight ______.
A．has been discovered recently
B．is known by everybody
C．still remains a mystery
D．has been known to scientists for years
Of course, scientists can't answer all of your questions. If we ask, "Why is the ocean full of salt？" scientists will say that the salt comes from rocks. When a rock gets very hot or very cold, it cracks. Rain falls into the cracks. The rain then carries the salt into he earth and into the rivers. The rivers carry the salt into the ocean. But then we ask, "What happens to the salt in the ocean？ The ocean doesn't get more slat every year." Scientists are not sure about the answer to this question.
We know a lot about our world. But there are still many answers that we do not have, and we are curious.
The main idea of this passage is ______.
A．people feel cool when they are near a lake or river
B．scientists can explain everything we want to know
C．scientists can explain many things but not everything
D．the salt in the ocean comes from rocks
What do the scientists hope to do in the future？
A．Reduce sleep. B, Control the way people feel.
B．Control the way people perform.
C．Both B and
Passage One
Why don't birds get lost on their long migratory (迁徙) flight？ Scientists have puzzled over this question for many years. Now they are beginning to fill in the blanks.
Not long ago, experiments showed that birds rely on the sun to guide them during day-light hours. But what about birds that fly mainly by night？ Tests with artificial (人造的) stars have proved conclusively that certain night-flying birds are able to follow stars in their long-distance flights.
One such bird--a warbler--had spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use stars for guidance. The bird's cage was placed under an artificial star-filled sky at migration time. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the artificial stars caused a change in the direction of his flight.
Scientists think that warblers, when flying in daylight, use the sun for guidance. But stars are apparently their main means of directed flight in the night. What do they do when stars are hidden by clouds？ Apparently, they find their way by such landmarks as mountain ranges, coast lines and river courses. But when it is too dark to see these, the warblers circle helplessly, unable to find their way.
Which of the following is NOT true about migratory birds' flight？
A．Some birds fly mainly by night.
B．Some birds fly mainly by day.
C．Birds depend on the sun or stars to guide them.
D．Birds like to fly during daylight hours.
It is said that you need to exercise only twice a week in order to keep a sufficient level of physical fitness. Studies conducted by American scientists show that unexercised muscles lose their strength very quickly. After 48 to 72 hours, you must use the muscles again to reestablish the good physical effects. And what does that mean to you？ American scientists concluded that while daily exercise is most helpful, every other day, three days a week will keep a sufficient level of physical fitness.
Is it true that you burn more calories running one mile than walking the same distance？ The answer is no. The truth is that you use the same amount of energy whether you walk or run the mile, since in both cases you are moving the same weight the same distance. The speed does not matter. Of course, if you run rather than walk for 30 minutes, you will cover more distance, and therefore burn more calories.
If your breathing doesn't return to normal within minutes after you finish exercising, you have exercised too much. Five minutes or so after exercising, your breathing should be normal, your heart shouldn't be beating, and you shouldn't be exhausted.
If you want to reduce your fat in a specific part, you ______.
A．should exercise all the muscles of your body
B．should exercise that specific part of your body
C．should do stomach exercises
D．can do any kind of exercise
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