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发表于 2004-10-31 05:42
species.[D] To discuss the role of time lags and geographic expansion of species in species detection. Part BDirections: In the following article, some sentences have been removed. For Questions 41—45, choose the most suitable one from the list A—G to fit into each of the numbered blank. There are two extra choices, which do not fit in any of the gaps. Mark your answers on ANSWER SHEET 1.(10 points)If you fail to stoke a fire,its flames at some time will die.Yet the great fire of the Sun has burned nonstop for about 5000 million years with no apparent sign of going out.41).__People in many cultures regarded the Sun as a miraculous gift from the gods,quite different from earthly fires and therefore not likely to fail them unless the gods were made angry.We now know that the Sun will eventually burn away.42).__The Sun is composed of almost 75 per cent hydrogen and 25 per cent helium,plus much smaller amounts of oxygen,carbon,neon,nitrogen,magnesium,iron and silicon.It is known as a main sequence star,one that shines by burning hydrogen.43).__ Thus, the Sun is both burning fuel and creating it. As the hydrogen store diminishes,its stock of helium grows.The light and heat coming now from the Sun were actually produced in its core many millions of years ago.When we burn fuel on a fire,we are converting matter—wood or coal—partly into energy.The more efficient the fire,the more heat it produces.44).__This loss of hydrogen is slight when compared to the Sun?s enormous bulk.Even though it is composed of light gas,the Sun weighs some 300,000 times as much as the Earth.And it loses about 4 million tones of matter every second.45).__If that sounds depressing,take heart!If the Sun switched off its power tomorrow,it would be ten million years years before its surface cooled sufficiently for anybody on Earth to feel 试题(二) 第11页the chill.In 5000 million years? time,humans may well have found an answer to impending doom. [A] We,one earth,soak up a tiny fraction—perhaps one?hundred?millionth—of the Sun?s vast energy.The rest of its awesome output of heat and light vanishes beyond the planets and into space.[B] Scientists predict that the Sun has enough hydrogen to keep the fire going for another 5000 million years,about as long again as it has already burned. It will then gradually,like the heart of a dying fire,change color,turning in this case from white to yellow and red until,finally,as a black dwarf,it disappears from sight.[C] The Earth?s atmosphere,which normally shields it from the intense heat of the Sun,would drift away. Then the Earth’s oceans would boil and vanish in steam.Without the cooling effect of its atmosphere and oceans,Earth itself would turn into a massive ball of fire.[D] At the Sun’s heart,the hydrogen was once compressed with such force that it started a nuclear reaction.In this giant furnace,the hydrogen is converted by nuclear fusion into another combustible gas,helium,in a reaction similar to that in an H?bomb.[E] Before its great fire dies,the Sun will turn into a red giant,bloated(swollen)to about 100 times its current size.It will swallow up Mercury and then Venus,the nearest to it planets.[F] The Sun is an extremely efficient furnace,but even so the helium it generates to keep the great fire going is only 92.3 per cent of the hydrogen it burns.The other 7.7 per cent vanishes in several forms of energy,mainly heat,light and X?rays.[G] Tests show that its temperature fluctuates.Since 1979,it seems to have cooled by one?tenth of per cent,but that is not a sign that the great fire is going out.Space scientists believe that,because of the Sun’s volatile nature,this minor change may soon be reversed. Part CDirections: |
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