托福阅读真题Official 51 Passage 2（七）

托福阅读真题Official 51 Passage 2（七）

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 13 of 14

Look at the four squares[■]that indicate where the following sentence could be added to the passage.Venus may not have always been this way.Where would the sentence best fit?Click on a square[■]to add the sentence to the passage.

正确答案：B

题目详解

题型分类：插入句子题

题干分析：考察句子和句子之间的衔接判断，待插入句意思是：金星可能并不总是这样，可以预测前后信息反向。

选项分析：

B选项，前面讲金星缺少水，选项后方讲金星以前有水，和地球水一样多，所以符合预测，B正确。

A选项，选项前面讲地质活动活跃，后面讲缺少水，只是金星的两个特点，构成不了反义。

C选项，选项后方和前面是解释关系，不是反义。

D选项，选向后方和前面顺成都讲失去水，不是反义。

Question 14 of 14

Directions:An introductory sentence for a brief summary of the passage is provided below.Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage.Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.This question is worth 2 points.

Over time,the movement of surface fluids has greatly changed Venus and Earth.

Answer Choices:

A.

Although Venus is about the same size as Earth,its greater volcanic activity has added considerably to carbon dioxide levels in its atmosphere.

B.

Because Venus lost the water it originally had,most of its carbon dioxide remained in its atmosphere,causing the planet to become very warm.

C.

Like Venus,Earth has an atmosphere,but Earth’s atmosphere has far more oxygen and nitrogen than does the atmosphere of Venus.

D.

On Earth,the dominance of river valley landscapes and the existence of life are due to the planet’s hydrologic cycle.

E.

On Earth,chemical reactions involving fluids remove carbon dioxide from the atmosphere by giving rise to carbonate rocks,and winds energized by gravity flow systems move fine particles from one place to another.

F.

The evaporation of liquid water from Earth’s surface is largely limited by the life forms that have developed,particularly the vegetation.

正确答案：BDE

题目详解

题型分类：文章总结题

题干分析：选择概括性的正确选项。

选项分析：

B选项，对应第二段最后一句。

D选项，对应第三段内容，尤其是正数第六句以及倒数第二句。

E选项，对应第三段内容的倒数第四句话，以及倒数第七句话。

A选项，前半部分对应第二段第二句正确，后半部分对应第二段倒数第三句，不正确，文章并没有提到金星火山活动比地球更频繁。

C选项，对应第三段最后一句，这句话只是在表达地球上有丰富的氧气与氮气而已，根本没有比较过金星和地球的氮氧含量。

F选项，对应最后一段最后两句话，有讲到生命，但没说气体蒸发受到生命的限制。