In the 1920s, Aleksandr Oparin, a Russian biochemist, proposed and developed the idea that life originated in the warm, watery environment of early Earth's surface, under anatmospheremostly composed ofmethane. The early seas were believed by Oparin to be rich in simpleorganicmolecules, which reacted to form morecomplexmolecules,eventuallyleadingto proteins and life. Then, almost 30 years after Oparin published his ideas, StanleyMillerdemonstrated thataminoacids, the building blocks of the proteins necessary for life, could form under conditions thought toprevailon early Earth. Miller's experiment waselegant. He passed electric dischargesthrougha mixture ofmethane,hydrogen,ammonia, and steam, and when he analyzed the results, found that he had madeaminoacids. The discharges were a proxy forlightning, the gas mixture aneducatedguess about what the earlyatmospheremay have been like. Aminoacids cannotreplicatethemselves, and are not themselves alive. Nevertheless, this experiment has long been recognized as alandmarkfor understanding aprocessthat must have been one of the important steps in theevolutionof life on Earth, the naturalsynthesisofaminoacids. However, it now seems likely thatMiller's experiments may not be directlyapplicableto the events of the early Archean (that is, early in the geologic eon that lasted from Earth'sformationuntil about 2.5 billion years ago).

One of the problems hindering understanding of theoriginof life is thatenvironmentalconditions on early Earth are not known with any certainty. It is possible to make only reasoned estimates. For example, for somefairlylong period of time afterformation, perhaps as much as several hundred million years, the surface must have been much hotter than it is today. Continued impacts of meteorites, large and small, would have added further heat energy, and in the earliest part of Earthhistorythe larger impacting bodies may have brokenthroughthe coolingcrusttoexposeunderlyingmoltenmaterial. Large quantities ofvolcanicgases would have been released into theatmosphereas lavas erupted onto the surface, producing a greenhouse effect far moreseverethan anything likely to result from humanactivity. It is quite possible that the earlyatmospherewas many times asdenseas today's, and that the seas and oceans were hot. Some haveevensuggested that because of the highatmosphericpressure, the oceans could have been hotter than the boiling point of water today. However, life as we know it is quitesensitivetotemperature, and no modern organisms are known tosurvivemuch above 100℃. It isunlikelythat life became established until surface temperatures had decreased to thislevel, orlower.

Although we do not know theprecisecompositionof the earlyatmosphere, there has been enough progress made on thissubjectin recent years that it is possible to say with some certainty that the methane-richcompositionenvisioned by Oparin, and the methane-ammonia-hydrogen mixture used byMillerin his experiments, are probably not veryrealistic. Based on studies of our closest neighbor planets, Mars andVenus, and also consideringevidencefrom Earth'ssedimentaryrocks, it seems probable that Earth's earlyatmospherewas rich incarbondioxiderather thanmethane. On both Mars andVenus,carbondioxideis by far the mostabundantgas in theatmosphere. On Earth it is aminorconstituent. But there is anenormousamountof thiscompoundburied in thesedimentaryrocks of Earth'scrust, enough so that, if it were all released, ouratmospherewould be much more like those of our neighboring planets. How didcarbondioxidegas end up in thecrust? The answer lies in what geologistsreferto as thecarboncycle. Throughaseriesof chemical reactions,carbondioxidefrom theatmospherefinds itself, in dissolved form, in the oceans. In seawater it combines withcalciumtoprecipitateascalciumcarbonate, the mainconstituentoflimestone. Over geologic time so muchcarbondioxidefrom theatmospherehas beenconvertedtolimestonein thisfashionthat there is more than 100,000 times as much stored aslimestoneas there is in theatmosphere.

Questions:
question_stem: According to paragraph 1, why were Miller's experimental results significant?
choices: tag: A
text: They explained the chemical composition of Earth during the early Archean.

tag: B
text: They provided evidence of the importance of electrical charges on early Earth.

tag: C
text: They showed how a key step in the evolution of life on Earth might have occurred.

tag: D
text: They demonstrated that amino acids could be created only in the presence of methane gas.
answer_key: C
explanation: 答案 C。根据题干信息 Miller's experimental results significant 定位到倒数第二句，this experiment has long been recognized as a landmark for understanding a process that must have been one of the important steps in the evolution of life on Earth, the natural synthesis of amino acids，landmark 替换了题目中的 significant，大意为这个实验结果对于理解一个过程很关键，即 amino acids 的自然合成是 evolution of life on Earth 的重要一步。所以答案选 C。

question_stem: According to paragraph 1, in setting up his experiment, Miller assumed each of the following to be true EXCEPT:
choices: tag: A
text: Earth's early atmosphere probably included hydrogen and ammonia.

tag: B
text: Lightning probably occurred on early Earth.

tag: C
text: Electrical discharges took place deep below early Earth's surface.

tag: D
text: Water was present in the early Earth.
answer_key: C
explanation: 答案 C。题干问哪个不是 Miller 设置实验的前提假设。考点信息范围从段落第三句话开始，A、D 对应到第五句话：He passed electric discharges through a mixture of methane, hydrogen, ammonia, and steam, and when he analyzed the results, found that he had made amino acids，其中 D 选项的 water 对应的就是 steam；B 对应第六句话，The discharges were a proxy for lightning, the gas mixture an educated guess about what the early atmosphere may have been like。C 选项未提及闪电电力释放的位置。

question_stem: In paragraph 1,why does the author remark that "Amino acids cannot replicate themselves, and are not themselves alive"?
choices: tag: A
text: To suggest that other researchers were unable to reproduce Miller's experimental results

tag: B
text: To establish the fact that Miller's experiment did not fully explain the origins of life

tag: C
text: To point out an error in Miller's educated guess about what the early atmosphere was like

tag: D
text: To support a statement about the importance of Miller's experiment
answer_key: B
explanation: 答案 B。定位题干信息，结合前文就近2句进行理解。定位句子大意为：Amino acids 不能自我复制也不是活的；而前文在介绍 Amino acids 合成的过程，后文紧接着又是转折关系 Nevertheless，表示了这个实验的意义在于表明了生命起源的过程肯定是存在 Amino acids 天然合成这一步骤的。所以作者提到 Amino acids 不能自我复制以及不是活跃的目的指出了：尽管这个实验对于理解生命起源具有重大意义，但是这个实验对于完全理解生命起源也有一定缺陷；所以答案确定为是 B。

question_stem: The word "Nevertheless" in the passage is closest in meaning to
choices: tag: A
text: In spite of this

tag: B
text: It is obvious that

tag: C
text: By comparison

tag: D
text: Therefore
answer_key: A
explanation: 答案 A。Nevertheless：然而、虽然如此；表转折关系。

question_stem: Paragraph 2 supports which of the following ideas about life on Earth?
choices: tag: A
text: Life on Earth probably first became established below the surface.

tag: B
text: Life probably did not get established until several hundred million years after Earth's formation.

tag: C
text: The earliest forms of life to become established were probably more sensitive to temperature than modern organisms are.

tag: D
text: Meteorites were probably responsible for bringing the first organisms to Earth.
answer_key: B
explanation: 答案 B。根据题干信息 life on Earth 定位到段落前三句以及倒数最后两句，for some fairly long period of time after formation, perhaps as much as several hundred million years, the surface must have been much hotter than it is today，地球形成几亿年后一直处于高热状态；no modern organisms are known to survive much above 100℃. It is unlikely that life became established until surface temperatures had decreased to this level, or lower.高温超过100度，没有生命可以幸存；生命的存在的时间应该是 several hundred million years later，surface temperatures 降低之后。

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: Because we still know little about the precise composition of Earth's early atmosphere, we cannot evaluate Oparin's theory or Miller's methane-ammonia-hydrogen experiment.

tag: B
text: Current knowledge suggests that Oparin and Miller were wrong about the composition of Earth's early atmosphere.

tag: C
text: We now know that it was very unrealistic of Oparin and Miller to think that they could identify the precise methane-rich mixture that made up the early atmosphere.

tag: D
text: Recent progress in understanding Earth's early history indicates that both Oparin and Miller clearly envisioned the precise atmospheric composition of early Earth.
answer_key: B
explanation: 答案 B。句子考察核心转折关系，主要大意为：虽然我们不知道早起大气层确切的组成，但是有进展的是我们能确定 Oparin 和 Miller 对大气层的构想是有不切实际的。A 首先逻辑关系不符合，其次 we cannot evaluate 也与原句相反了；C 较容易被干扰，they could identify the precise methane-rich mixture 这部分表述与原文不符，原文意思不是对于精准的 methane-rich mixture 的认定，而是他们觉得大气的组成是 methane-rich composition 或者是 methane-ammonia-hydrogen mixture；D 中 indicates that both Oparin and Miller clearly envisioned the precise atmospheric composition of early Earth 与原文相反，当前的理解提升是认为这两人的设想不合理。

question_stem: According to paragraph 3, what is the significance of the fact that the rocks of Earth's crust now contain an enormous amount of calcium carbonate?
choices: tag: A
text: It explains why Oparin and Miller believed that Earth's early atmosphere was methane rich.

tag: B
text: It supports the idea that the atmosphere of early Earth was rich in carbon dioxide.

tag: C
text: It helps explain where the carbon dioxide in Earth's atmosphere came from.

tag: D
text: It provides evidence about the likely composition of the crust of early Earth.
answer_key: B
explanation: 答案 B。根据题干信息 Earth's crust now contain an enormous amount of calcium carbonate 定位到本段最后三句，来自大气层里的 carbon dioxide 溶解在海里，在海里它与 calcium 结合形成 calcium carbonate，也就是 limestone 石灰石的主要成分，非常多的 carbon dioxide 形成石灰石，储存在石灰石里面的 carbon dioxide 是大气层中的10万倍，也就是说曾今大气层里的 carbon dioxide 含量很高，最后大都转化成了 calcium carbonate。所以答案选 B。