托福閱讀真題Official 51 Passage 2（六）

托福閱讀真題Official 51 Passage 2（六）

Surface Fluids on Venus and Earth

A fluid is a substance,such as a liquid or gas,in which the component particles(usually molecules)can move past one another.Fluids flow easily and conform to the shape of their containers.The geologic processes related to the movement of fluids on a planet’s surface can completely resurface a planet many times.These processes derive their energy from the Sun and the gravitational forces of the planet itself.As these fluids interact with surface materials,they move particles about or react chemically with them to modify or produce materials.On a solid planet with a hydrosphere the combined mass of water on,under,or above a planet’s surface and an atmosphere,only a tiny fraction of the planetary mass flows as surface fluids.Yet the movements of these fluids can drastically alter a planet.Consider Venus and Earth,both terrestrial planets with atmospheres.

Venus and Earth are commonly regarded as twin planets but not identical twins.They are about the same size,are composed of roughly the same mix of materials,and may have been comparably endowed at their beginning with carbon dioxide and water.However,the twins evolved differently,largely because of differences in their distance from the Sun.With a significant amount of internal heat,Venus may continue to be geologically active with volcanoes,rifting,and folding.However,it lacks any sign of a hydrologic system(water circulation and distribution):there are no streams,lakes,oceans,or glaciers.Space probes suggest that Venus may have started with as much water as Earth,but it was unable to keep its water in liquid form.Because Venus receives more heat from the Sun,water released from the interior evaporated and rose to the upper atmosphere where the Sun’s ultraviolet rays broke the molecules apart.Much of the freed hydrogen escaped into space,and Venus lost its water.Without water,Venus became less and less like Earth and kept an atmosphere filled with carbon dioxide.The carbon dioxide acts as a blanket,creating an intense greenhouse effect and driving surface temperatures high enough to melt lead and to prohibit the formation of carbonate minerals.Volcanoes continually vented more carbon dioxide into the atmosphere.On Earth,liquid water removes carbon dioxide from the atmosphere and combines it with calcium,from rock weathering,to form carbonate sedimentary rocks.Without liquid water to remove carbon from the atmosphere,the level of carbon dioxide in the atmosphere of Venus remains high.

Like Venus,Earth is large enough to be geologically active and for its gravitational field to hold an atmosphere.Unlike Venus,it is just the right distance from the Sun so that temperature ranges allow water to exist as a liquid,a solid,and a gas.Water is thus extremely mobile and moves rapidly over the planet in a continuous hydrologic cycle.Heated by the Sun,the water moves in great cycles from the oceans to the atmosphere,over the landscape in river systems,and ultimately back to the oceans.As a result,Earth’s surface has been continually changed and eroded into delicate systems of river valleys—a remarkable contrast to the surfaces of other planetary bodies where impact craters dominate.Few areas on Earth have been untouched by flowing water.As a result,river valleys are the dominant feature of its landscape.Similarly,wind action has scoured fine particles away from large areas,depositing them elsewhere as vast sand seas dominated by dunes or in sheets of loess(fine-grained soil deposits).These fluid movements are caused by gravity flow systems energized by heat from the Sun.Other geologic changes occur when the gases in the atmosphere or water react with rocks at the surface to form new chemical compounds with different properties.An important example of this process was the removal of most of Earth’s carbon dioxide from its atmosphere to form carbonate rocks.However,if Earth were a little closer to the Sun,its oceans would evaporate;if it were farther from the Sun,the oceans would freeze solid.Because liquid water was present,self-replicating molecules of carbon,hydrogen,and oxygen developed life early in Earth’s history and have radically modified its surface,blanketing huge parts of the continents with greenery.Life thrives on this planet,and it helped create the planet’s oxygen and nitrogen-rich atmosphere and moderate temperatures.

Question 11 of 14

Why does the author point out that on Earth“gases in chemical compounds”?

A.To explain why scientists believe that few areas on Earth have been untouched by flowing water

B.To identify one of several ways in which the movement of fluids can affect the surface of a planet

C.To provide evidence that fluid movements are caused by gravity flow systems energized by the Sun

D.To identify an effect of wind scouring fine particles away from large areas

正確答案：B

題目詳解

題型分類：修辭目的題

題干分析：考察句子之間的關系。

選項分析：

Other geologic changes occur：其他地質(zhì)改變發(fā)生，說明在前文也有提到過地質(zhì)變化，前一句講wind action has scoured fine particles away from large areas，因此，風力作用是另一種地質(zhì)變化，所以，wind action和other geologic changes都是流體改變地貌的方式，與B選項一致。

A選項，層次錯亂。

C選項，高干擾度選項，定位句和前一句是并列關系，而且分別給出了地質(zhì)變化的原因，所以選項中解釋關系錯誤。

D選項，和前文是并列，不是解說關系。

Question 12 of 14

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?Incorrect choices change the meaning in important ways or leave out essential information.

A.Life on Earth is responsible for many changes to the planet’s surface,including blankets of greenery.

B.Self-replicating molecules of carbon,hydrogen,and oxygen led to the development of life early in Earth’s history.

C.The presence of water made it possible for life to develop early in Earth’s history and to significantly change its surface.

D.Early in life’s history on Earth,self-replicating molecules of carbon,hydrogen,and oxygen beganblanketing the surface in greenery.

正確答案：C

題目詳解

題型分類：簡化句子題

原文分析：考察識別主干內(nèi)容和核心邏輯的能力，提煉重點信息：self-replicating molecules...developed life early in Earth's history and have rapidly modified its surface,blanketing huge parts of the continents with greenery，意思是分子發(fā)展成生命并且改變地球表面，最后結(jié)果是使大陸的大部分被綠色植物覆蓋。

選項分析：

C選項，提煉重點信息：...made it possible for life to develop early in Earth's history and to significantly change its surface，與原文重點信息一致。

A選項，提煉重點信息：Life on Earth is responsible for many changes to the planet's surface，邏輯錯誤，生命出現(xiàn)和地表改變是并列關系，不是因果。

B選項，提煉重點信息：Self-replicating molecules of carbon,hydrogen,and oxygen led to the development of life early in Earth's history，不全面，沒說最后的結(jié)果以及根本原因。

D選項，提煉重點信息：self-replicating molecules of carbon,hydrogen,and oxygen began blanketing the surface in greenery，主干信息缺失，選項沒說發(fā)展生命并且改變地球表面。

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