The deserts, which alreadyoccupyapproximatelya fourth of the Earth's land surface, have in recent decades been increasing at an alarming pace. Theexpansionof desert-like conditions into areas where they did notpreviouslyexist is calleddesertification. It has been estimated that anadditionalone-fourth of the Earth's land surface isthreatenedby thisprocess.

Desertificationisaccomplishedprimarilythroughthe loss of stabilizing naturalvegetationand thesubsequentacceleratederosionof the soil by wind and water. In some cases theloosesoil is blown completely away, leaving a stony surface. In other cases, the finer particles may be removed, while the sand-sized particles are accumulated to formmobilehills or ridges of sand.

Evenin the areas thatretaina soil cover, thereductionofvegetationtypically results in the loss of the soil'sabilitytoabsorbsubstantialquantities of water. Theimpactof raindrops on theloosesoil tends totransferfineclayparticles into the tiniest soil spaces, sealing them and producing a surface that allows very little water penetration. Waterabsorptionis greatly reduced;consequentlyrunoffis increased, resulting inacceleratederosionrates. Thegradualdrying of the soil caused by itsdiminishedabilitytoabsorbwater results in the further loss ofvegetation, so that acycleofprogressivesurfacedeteriorationis established.

In some regions, the increase indesertareas is occurring largely as the result of atrendtoward drierclimaticconditions. Continuedgradualglobal warming has produced an increase in aridity for some areas over the past few thousand years. Theprocessmay beacceleratedinsubsequentdecades if global warming resulting fromairpollution seriously increases.

There is littledoubt, however, thatdesertificationin most areas resultsprimarilyfrom human activities rather than natural processes. Thesemiaridlands bordering the deserts exist in adelicateecologicalbalanceand are limited in theirpotentialtoadjustto increasedenvironmentalpressures. Expanding populations are subjecting the land to increasing pressures to provide them with food andfuel. In wet periods, the land may be able torespondto these stresses. During the dry periods that arecommonphenomenaalongthedesertmargins, though, thepressureon the land is often far inexcessof itsdiminishedcapacity, anddesertificationresults.

Fourspecificactivities have been identified asmajorcontributors to thedesertificationprocesses: overcultivation, overgrazing, firewood gathering, and overirrigation. Thecultivationof crops has expanded into progressively drier regions aspopulationdensities have grown. These regions are especially likely to have periods ofseveredryness, so that crop failures arecommon. Since the raising of most crops necessitates thepriorremovalof the naturalvegetation, crop failures leaveextensivetracts of landdevoidof a plant cover andsusceptibleto wind and watererosion.

The raising oflivestockis amajoreconomicactivityinsemiaridlands, where grasses are generally thedominanttype of naturalvegetation. The consequences of anexcessivenumber oflivestockgrazingin an area are thereductionof thevegetationcover and the trampling and pulverization of the soil. This is usually followed by the drying of the soil andacceleratederosion.

Firewood is thechieffuelused for cooking and heating in many countries. The increased pressures of expanding populations have led to theremovalof woody plants so that many cities and towns are surrounded by large areas completely lacking in trees and shrubs. The increasing use of dried animalwasteas asubstitutefuelhas also hurt the soil because thisvaluablesoil conditioner andsourceof plant nutrients is no longer being returned to the land.

The finalmajorhuman cause ofdesertificationis soilsalinizationresulting from overirrigation. Excesswater fromirrigationsinks down into the water table. If nodrainagesystem exists, the water table rises, bringing dissolved salts to the surface. The water evaporates and the salts are left behind, creating a whitecrustallayerthat preventsairand water from reaching theunderlyingsoil.

Theextremeseriousness ofdesertificationresults from thevastareas of land and thetremendousnumbers of people affected, as well as from the great difficulty of reversing orevenslowing theprocess. Once the soil has been removed byerosion, only thepassageof centuries ormillenniawill enable new soil to form. In areas whereconsiderablesoil still remains, though, a rigorously enforced program of land protection and cover-crop planting may make it possible toreversethe presentdeteriorationof the surface.

Questions:
question_stem: The word “threatened” in the passage is closest in meaning to
choices: tag: A
text: restricted

tag: B
text: endangered

tag: C
text: prevented

tag: D
text: rejected
answer_key: B
explanation: threaten: 威胁。A 是限制；B 是危及；C 是阻止，避免；D 是拒绝。和选项 B 呼应，所以正确答案是 B。对应原文 It has been estimated that an additional one-fourth of the Earth's land surface is threatened by this process.句子说大约1/4的土地收到这个过程的 threaten。ACD 代入后发现句子不通，只有 B 最合适。

question_stem: According to paragraph 3, the loss of natural vegetation has which of the following consequences for soil?
choices: tag: A
text: Increased stony content

tag: B
text: Reduced water absorption

tag: C
text: Increased numbers of spaces in the soil

tag: D
text: Reduced water runoff
answer_key: B
explanation: 用 vegetation 定位到第一句话： Even in the areas that retain a soil cover, the reduction of vegetation typically results in the loss of the soil's ability to absorb substantial quantities of water.这句话中明确说了，植被的减少导致土壤是去吸收大量水分的能力。这就对应了 B 选项。A 在原文未提及，错；文中并没有说"spaces in the soil" increase 了，所以 C 错；D 选项和文中内容相反，所以错。

question_stem: The word “delicate” in the passage is closest in meaning to
choices: tag: A
text: fragile

tag: B
text: predictable

tag: C
text: complex

tag: D
text: valuable
answer_key: A
explanation: delicate: 易碎的，纤弱的。和选项 A 呼应。B 是可预测的；C 是复杂的；D 是有价值的。对应原文 he semiarid lands bordering the deserts exist in a delicate ecological balance and are limited in their potential to adjust to increased environmental pressures.这句话中有并列关系，从 and 之后的 limited 可以推测出 delicate 应该是个负面的词，D 首先被排除，D 是正面词汇；BC 是中性词，不选。只有 A 最合适。

question_stem: According to paragraph 5, in dry periods, border areas have difficulty
choices: tag: A
text: adjusting to stresses created by settlement

tag: B
text: retaining their fertility after desertification

tag: C
text: providing water for irrigating crops

tag: D
text: attracting populations in search of food and fuel
answer_key: A
explanation: 用 dry periods 定位到最后一句： During the dry periods that are common phenomena along the desert margins, though, the pressure on the land is often far in excess of its diminished capacity, and desertification results.这里强调了压力大于承受能力，pressure 对应 A 选项中的 stresses，A 正确。选项 B 中的 fertilit 和 C 选项中的 irrigatting crops 没有被提及过, 所以 B 和 C 错。 选项 D 中的"attracting populations"也没有被提及到。

question_stem: The word “progressively” in the passage is closest in meaning to
choices: tag: A
text: openly

tag: B
text: impressively

tag: C
text: objectively

tag: D
text: increasingly
answer_key: D
explanation: progressively: 渐进的，日益增多的。和选项 D 呼应。A 是公开地；B 是令人难忘地；C 是客观地。对应原文 The cultivation of crops has expanded into progressively drier regions as population densities have grown.这句话会所：当人口密度增加的时候，农作物的种植已经 progressively 扩展到更加干燥的地方了。

question_stem: According to paragraph 6, which of the following is often associated with raising crops?
choices: tag: A
text: Lack of proper irrigation techniques

tag: B
text: Failure to plant crops suited to the particular area

tag: C
text: Removal of the original vegetation

tag: D
text: Excessive use of dried animal waste
answer_key: C
explanation: 根据 raising crops 定位到最后一句： Since the raising of most crops necessitates the prior removal of the natural vegetation, crop failures leave extensive tracts of land devoid of a plant cover and susceptible to wind and water erosion.这里说了，种植农作物需要事先移除自然植被，农作物的失败会让土地没有覆盖并且遭受到风和水的侵蚀。呼应 C 选项。

question_stem: The phrase “devoid of” in the passage is closest in meaning to
choices: tag: A
text: consisting of

tag: B
text: hidden by

tag: C
text: except for

tag: D
text: lacking in
answer_key: D
explanation: devoid of: 没有，缺乏。和选项 D 呼应。A 是由…组成；B 是被…隐藏；C 是除了；D 是缺乏。对应原文 Since the raising of most crops necessitates the prior removal of the natural vegetation, crop failures leave extensive tracts of land devoid of a plant cover and susceptible to wind and water erosion.原句有着明显的顺承关系，用 and 连接着，所以农作物失败留下土地 devoid of 植被覆盖，然后就遭受了侵蚀。这样的顺承关系可以退出，devoid of 应该是否定的表达，没有了覆盖才会收到侵蚀。所以正确答案是 D。

question_stem: According to paragraph 9, the ground’s absorption of excess water is a factor in desertification because it can
choices: tag: A
text: interfere with the irrigation of land

tag: B
text: limit the evaporation of water

tag: C
text: require more absorption of air by the soil

tag: D
text: bring salts to the surface
answer_key: D
explanation: 用 excess water 定位到第二句，但这句话并没有说出原因，往后看，后一句话出答案： If no drainage system exists, the water table rises, bringing dissolved salts to the surface.这句话明确说了，如果没有排水系统存在，地下水位会升高，把盐带到地表，和 D 选项呼应， 所以正确答案是 D。ABC 均未提及。

question_stem: All of the following are mentioned in the passage as contributing to desertification EXCEPT
choices: tag: A
text: soil erosion

tag: B
text: global warming

tag: C
text: insufficient irrigation

tag: D
text: the raising of livestock
answer_key: C
explanation: 选项 A 的 "soil erosion" 在第二段第一句话有提及： Desertification is accomplished primarily through the loss of stabilizing natural vegetation and the subsequent accelerated erosion of the soil by wind and water.选项 B, "global warning" 在第四段第二句所提及： Continued gradual global warming has produced an increase in aridity for some areas over the past few thousand years 选项 D, "raising of livestock" 在第七段倒数第二句所提及： The consequences of an excessive number of livestock grazing in an area are the reduction of the vegetation cover and the trampling and pulverization of the soil.选项 C 的 irrigation 在第9段提及，但是 C 说反，原文说 overirrigation 而不是 insufficient，所以错，选： The final major human cause of desertification is soil salinization resulting from overirrigation.

question_stem: 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.
choices: tag: A
text: Desertification is a significant problem because it is so hard to reverse and affects large areas of land and great numbers of people.

tag: B
text: Slowing down the process of desertification is difficult because of population growth that has spread over large areas of land.

tag: C
text: The spread of deserts is considered a very serious problem that can be solved only if large numbers of people in various countries are involved in the effort.

tag: D
text: Desertification is extremely hard to reverse unless the population is reduced in the vast areas affected.
answer_key: A
explanation: 这句话的主要逻辑是并列，并列的几个对象缺一不可：1）广大的土地；2）大量的人口受到影响；3）减缓或者倒转这个过程很困难。选项 A 的逻辑和原文呼应，并且并列的对象都包括在选项中，所以 A 正确；B 选项的结果部分错误，原句是 desertification 的严峻形势，这里变成了减缓过程，而且原因部分没将3个部分都涵盖；C 的 solve 这部分内容原句没有，纯属瞎编；D 的 unless 结构和原文的逻辑不符。

question_stem: It can be inferred from the passage that the author most likely believes which of the following about the future of desertification?
choices: tag: A
text: Governments will act quickly to control further desertification.

tag: B
text: The factors influencing desertification occur in cycles and will change in the future.

tag: C
text: Desertification will continue to increase.

tag: D
text: Desertification will soon occur in all areas of the world.
answer_key: C
explanation: 本题较难，需联系首尾两段。文章开篇便指出沙漠已经(already)覆盖地球四分之一的面积，并且近年来沙漠化以惊人速度扩张(at an alarming pace)，使得又一四分之一的土地(an additional one-fourth of the Earth's land surface)面临威胁。文章结尾时总结到：土壤一旦被腐蚀，只有数百甚至千年后(only the passage of centuries or millennia)才能新生； though 理解为"虽然"，作者意图说明虽然在那些土壤尚存的地方通过保护可能(may make it possible to reverse)可以制止恶化，但其实这句话还有言外之意，即那些已经腐蚀严重的地区前景就很不乐观了。综上，作者对沙漠化前景持悲观态度，认为沙化会继续扩张。因此选择 C。