Photographicevidencesuggests thatliquidwater once existed in greatquantityon the surface of Mars. Two types of flow features are seen:runoffchannels and outflow channels. Runoffchannels are found in the southern highlands. These flow features areextensivesystems—sometimes hundreds of kilometers in total length—of interconnecting, twisting channels thatseemtomergeinto larger, wider channels. Theybeara strong resemblance to river systems on Earth, and geologists think that they are dried-up beds of long-gone rivers that once carriedrainfallon Mars from the mountains down into the valleys. Runoffchannels on Mars speak of a time 4 billion years ago (the age of the Martian highlands), when theatmospherewas thicker, the surface warmer, andliquidwaterwidespread.

Outflow channels are probably relics ofcatastrophicflooding on Mars long ago.Theyappearonly inequatorialregions and generally do not formextensiveinterconnected networks.Instead, they are probably the paths taken by huge volumes of water draining from the southern highlands into the northern plains.The onrushing water arising from theseflashfloods

likely also formed theoddteardrop-shaped “islands” (resembling the miniature versions seen in the wet sand of our beaches at lowtide) that have been found on the plains close to the

ends of the outflow channels.Judging from the width and depth of the channels, the flow rates must have been trulyenormous—perhaps as much as a hundred times greater than the 105 tons persecondcarried by the great Amazon river.Flooding shaped the outflow channelsapproximately3 billion years ago, about the same times as the northernvolcanicplains formed. Theyappearonly inequatorialregions and generally do not formextensiveinterconnected networks. Instead, they are probably the paths taken by huge volumes of water draining from the southern highlands into the northern plains. The onrushing water arising from theseflashfloods

likely also formed theoddteardrop-shaped “islands” (resembling the miniature versions seen in the wet sand of our beaches at lowtide) that have been found on the plains close to the

ends of the outflow channels. Judging from the width and depth of the channels, the flow rates must have been trulyenormous—perhaps as much as a hundred times greater than the 105 tons persecondcarried by the great Amazon river. Flooding shaped the outflow channelsapproximately3 billion years ago, about the same times as the northernvolcanicplains formed.

Some scientistsspeculatethat Mars may have enjoyed an extended early Period during which rivers, lakes, and perhapsevenoceans adorned its surface. A 2003 Mars GlobalSurveyorimageshows whatmissionspecialists think may be adelta—a fan-shaped network of channels and sediments where a river once flowed into a larger body of water, in this case a lake filling acraterin the southern highlands. Other researchers goevenfurther, suggesting that thedataprovideevidencefor large open expenses of water on the early Martian surface. A computer-generated view of the Martian northpolarregionshows theextentof what may have been anancientocean covering much of the northern lowlands. The HellasBasin, which measures some 3,000 kilometersacrossand has a floor that lies nearly 9 kilometers below thebasin’s rim, is anothercandidatefor anancientMartian sea.

These ideasremaincontroversial. Proponents point to features such as the terraced “beaches” shown in oneimage, which couldconceivablyhave been left behind as a lake or ocean evaporated and the shoreline receded. But detractorsmaintainthat the terraces could also have been created by geologicalactivity, perhaps related to the geologic forces that depressed the Northern Hemisphere far below thelevelof the south, in which case they have nothing whatever to do with Martian water. Furthermore, Mars GlobalSurveyordatareleased in 2003seemtoindicatethat the Martian surface contains too fewcarbonaterock layers—layers containing compounds ofcarbonandoxygen—that should have been formed inabundancein anancientocean. Theirabsencesupports the picture of a cold, dry Mars that never experienced the extendedmildperiod required to form lakes and oceans. However, more recentdataimplythat atleastsome parts of theplanetdid in fact experience long periods in the past during whichliquidwater existed on the surface.

Aside from some small-scale gullies (channels) found since 2000, which areinconclusive, astronomers have no directevidenceforliquidwater anywhere on the surface of Mars today, and theamountof watervaporin the Martianatmosphereis tiny. Yetevensettingaside the unproven hints ofancientoceans, theextentof the outflow channels suggests that a huge totalvolumeof water existed on Mars in the past. Where did all the water go? The answer may be thatvirtuallyall the water on Mars is now locked in the permafrostlayerunder the surface, with more contained in theplanet’spolarcaps.

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

tag: B
text: separate

tag: C
text: straighten out

tag: D
text: combine
answer_key: D
explanation: merge: 合并。A 是扩张；B 是分开；C 是改正，扯平；D 是结合。所以 D 的 combine 正确。对应原文：These flow features are extensive systems--sometimes hundreds of kilometers in total length--of interconnecting, twisting channels that seem to merge into larger, wider channels. 原句说那些 interconnecting 的，twisting 的 channel 怎么样成为大的更宽的 channel，merge 应该有变大之意。expand 和 combine 可能正确，具体哪个对只能靠认识了。

question_stem: What does the discussion in paragraph 1 of runoff channels in the southern highlands suggest about Mars?
choices: tag: A
text: The atmosphere of Mars was once thinner than it is today.

tag: B
text: Large amounts of rain once fell on parts of Mars.

tag: C
text: The river systems of Mars were once more extensive than Earth’s.

tag: D
text: The rivers of Mars began to dry up about 4 billion years ago.
answer_key: B
explanation: B 以 runoff channels 和 southern highlands 做关键词定位至第三句，但问题问的是表明了什么，关键词所在句明显没说，接着本段又说了一大堆这种 channel 的特点，直到最后一句才说 speak of blabla，答案在最后一句，B 正确；A 和 D 说反了；C 没说。

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

tag: B
text: sites

tag: C
text: requirements

tag: D
text: sources
answer_key: A
explanation: A, relics: 遗迹，残余。A 是残余；B 是遗址；C 是要求；D 是资源。所以 A 的 remains 正确。

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

tag: B
text: small

tag: C
text: multiple

tag: D
text: familiar
answer_key: B
explanation: B miniature: 小型的，缩略图，小模型，所以 B 的 small 正确。先看单词本身，前半部分是个 mini，就是小的意思，迷你，所以会猜到跟小相关。A 是暂时的；C 是多重的，多样的；D 是熟悉的。

question_stem: In paragraph 2, why does the author include the information that 105 tons of water flow through the Amazon river per second?
choices: tag: A
text: To emphasize the great size of the volume of water that seems to have flowed through Mars’ outflow channels

tag: B
text: To indicate data used by scientists to estimate how long ago Mars’ outflow channels were formed

tag: C
text: To argue that flash floods on Mars may have been powerful enough to cause tear-shaped “islands” to form

tag: D
text: To argue that the force of flood waters on Mars was powerful enough to shape the northern volcanic plains
answer_key: A
explanation: A 修辞目的题，往前看，本句修词点之前有破折号，证明这个105 ton 是解释前面内容的。之前说那些 channel 的水流量非常大，然后马上出现了一个105 ton 的例子，所以答案是 A。

question_stem: According to paragraph 2, all of the following are true of the outflow channels on Mars EXCEPT:
choices: tag: A
text: They formed at around the same time that volcanic activity was occurring on the northern plains.

tag: B
text: They are found only on certain parts of the Martian surface.

tag: C
text: They sometimes empty onto what appear to have once been the wet sands of tidal beaches.

tag: D
text: They are thought to have carried water northward from the equatorial regions.
answer_key: C
explanation: C EXCEPT 题，排除法。A 的 volcanic activity 做关键词定位至最后一句，正确，不选；B 的 certain parts 与原文第二句的 equatorial regions 同义重合，正确，不选；C 的 beaches 做关键词定位至倒数第三句，但原文说洪水形成的小岛形状像海滩上的沙子，跟 C 说的不同，所以 C 错，选；D 的 northward 做关键词定位至第三句，再结合第二句，说明 D 正确，不选。

question_stem: All of the following questions about geological features on Mars are answered in paragraph 3 EXCEPT:
choices: tag: A
text: What are some regions of Mars that may have once been covered with an ocean?

tag: B
text: Where do mission scientists believe that the river forming the delta emptied?

tag: C
text: Approximately how many craters on Mars do mission scientists believe may once have been lakes filled with water?

tag: D
text: During what period of Mars’ history do some scientists think it may have had large bodies of water?
answer_key: C
explanation: D EXCEPT 题，排除法。A 的 regions 做关键词定位至第二句的后半部分和倒数第二句，正确，不选；B 的 delta 做关键词定位至原文第二句，正确，不选；C 的 craters 做关键词定位至第二句，但原文只说有 crater 被填满，没说几个，所以 C 错，选；D 的 large bodies of water 与原文第一句的 rivers, lakes and even oceans 同义重合，正确，不选。

question_stem: According to paragraph 3, images of Mars’ surface have been interpreted as support for the idea that
choices: tag: A
text: the polar regions of Mars were once more extensive than they are now

tag: B
text: a large part of the northern lowlands may once have been under water

tag: C
text: deltas were once a common feature of the Martian landscape

tag: D
text: the shape of the Hellas Basin has changed considerably over time
answer_key: B
explanation: D 排除法。A 的 polar 做关键词定位至倒数第二句，原文没说比现在更 extensive，错，不选；同时这句话说北部的 lowland 可能是个 ancient ocean，与 B 选项说的在水下是同义重现，所以 B 正确，选；C 的 deltas 做关键词定位至第二句，原文没说 common，错，不选；D 的专有名词定位至最后一句，没说变化，错，不选。

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: But detractors argue that geological activity may be responsible for the water associated with the terraces.

tag: B
text: But detractors argue that the terraces may have been formed by geological activity rather than by the presence of water.

tag: C
text: But detractors argue that the terraces may be related to geological forces in the Northern Hemisphere of Mars, rather than to Martian water in the south.

tag: D
text: But detractors argue that geological forces depressed the Northern Hemisphere so far below the level of the south that the terraces could not have been formed by water.
answer_key: B
explanation: B 从句套并列句，原句说 detractors 认为 terrace 是由地质活动，而非水的原因形成的。A 错，水和地质活动是对等的两面，不是因果关系；B 正确；C 的 southern 和 northern 偷换原文概念，内容乱拼，错；D 与 A 的错误类似，原文的地质活动和水是相对立的关系，不是这里的 so that 引导的因果关系，错。

question_stem: According to paragraph 4, what do the 2003 Global Surveyor data suggest about Mars?
choices: tag: A
text: Ancient oceans on Mars contained only small amounts of carbon.

tag: B
text: The climate of Mars may not have been suitable for the formation of large bodies of water.

tag: C
text: Liquid water may have existed on some parts of Mars’ surface for long periods of time.

tag: D
text: The ancient oceans that formed on Mars dried up during periods of cold, dry weather.
answer_key: B
explanation: B 以2003 Global Surveyor data 做关键词定位至第四句，说没有 carbonate，而 carbonate 是大洋中经常有的，也就是说火星上没有 ocean，而且下一句说 cold dry 火星，所以 B 正确。

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

tag: B
text: features

tag: C
text: arguments

tag: D
text: effects
answer_key: A
explanation: hint: 暗示，示意。A 是线索；B 是特征；C 是论证；D 是效果。所以 A 的 clue 正确。对应原文：Yet even setting aside the unproven hints of ancient oceans, the extent of the outflow channels suggests that a huge total volume of water existed on Mars in the past.原句说把未经证实的什么搁置，之前说除了2000年发现的一些 gully 之外，astronomers 没有直接证据，紧接着说把未经证实的什么搁置，肯定是证据之类的意思，B 特点 C 争论 D 影响都不对。