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  3. 共用题干 Where Did the Earth's Water Come From?Earth is located within the"sno...

共用题干 Where Did the Earth's Water Come From?Earth is located within the"sno...

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问题 共用题干
Where Did the Earth's Water Come From?

Earth is located within the"snow line"of the solar system,the region closest to the Sun where H2 0 is primarily in liquid or gaseous form,if at all.______(46)The snow line phenomena is reflected in the water content of planets like Mercury,Venus,and Mars. Water is absent on Mercury.On Venus,H2O only exists as a trace element in the atmosphere.Mars only has a thin veneer of ice in its polar regions.In general,water is rare within the snow line.
Why does Earth have so much water relative to the other inner planets?71%of the surface is covered in the oceans,more than half of which is deeper than 3,000 meters,with an approximate total volume of 1.3 billion cu km.______(47)
There are various theories as to where all the Earth's water came from,but several theories have fared better than the others.______(48)When the Earth was in the process of forming,with a radius just 40%smaller than at present,it would have had enough gravity to hold on to a tenuous atmosphere with water vapor. The first water vapor on the planet would have come from the planet's internals,where volatile(low weight)chemicals would have a tendency to float to the top,and heavy chemicals(iron and nickel)would sink.
Though the first of Earth's water came about through volcanism,this alone probably didn't produce enough to form stable pools on the surface.______(49)Comparing the isotope ratios of water on Earth and water from comets and asteroids has revealed that the majority of the Earth's water comes from asteroids.
Throughout its history,Earth's water has increased in volume due to biological processes. In the early seas of Earth,hydrogen sulfide would have been in great supply,which,when reacted with carbon dioxide from the atmosphere during photosynthesis in sulfide-reducing bacteria,would have produced hydrogen,sulfur,and water.______(50)
选项 ________（48）
A: More water was added to the planet during several hypothesized large impacts from asteroids from the outer asteroid belt.
B:We know that the oceans existed as early as 100 million years after the formation of the Earth.
C:Still,the oceans only make up 0.023%of the Earth's total mass.
D:Astronomers say Earth s water may have come from space.
E:The snow line is located in the outerasteroid belt,between the orbits of Mars and Jupiter.
F: Many geologists believe that the majority of Earth's water generated through this process.
答案B
解析文中第一段第一句提到了snow line，接下来的依据肯定还是要对snow line进行进一步说明，而选项中，只有E选项提到了snow line，所以选择E。
文中第二段主要说明地球上的水占的比重，而只有C选项是对这一主题进行说明，所以选择C。
根据后一句When the Earth was in the process of forming可以得知，该段是在讲述地球的形成历史，B选项We know that the oceans existed as early as 100 million years after the formation of the Earth也是在讲述地球形成的历史，所以选择B。
后一句：Comparing the isotope ratios of water on Earth and water from comets and asteroids has revealed that the majority of the Earth's water comes from asteroids．通过把地球上水的同位素和小行星以及彗星上的水的同位素进行比较，发现地球上的水主要来自小行星。可以推断出，该句要讲述地球上的水与太空中小行星的关系，所以A选项正确。
文中的最后一段主要是讲地球上水的来源。前一句：In the early seas of Earth,hydrogen sulfide would have been in great supply,which,when reacted with carbon dioxide from the atmosphere during photosynthesis in sulfide-reducing bacteria,would have produced hydrogen , sulfur , and water．在地球早期的海洋中，硫氢化合物非常丰富，在去硫细菌的光合作用的影响下，它与大气层中的二氧化碳产生化学反应，产生大量的氢、硫，以及水。这是地球自身产生的水，而F选项Many geologists believe that the majority of Earth's water generated through this process(许多地理学家认为地球上水的产生主要通过这一进程实现）正好承接前一句，所以F选项正确。
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