Until about the late 1980s, most scientists believed that life wasrestrictedto the top few meters of the soil or ocean sediments. The few reports of organisms being recovered from great depthswithinEarth were dismissed ascontaminationwithmaterialfrom the surface layers. Twotechnicaldevelopments changed this view. The first was thedevelopmentof drilling techniques that gaveconfidencethat samples could be retrieved from depth withoutcontamination. Samples were recovered using a diamond-studdeddrillbit that headed a great length of rotatingsteelpipefrom a drilling derrick. A concentrated tracermaterialwas added to the lubricatingfluidso that when a deepsampleof rock was removed, any contaminatedmaterialcould be identified and cut away to leave a pristinesampleof rock from deepwithinEarth. Theseconddevelopmentwas theadventof techniques for identifying microorganisms without having to grow them inculture. All organismscontainDNA, and their presence can be revealed by dyes that eitherstainDNA directly or can be attached to nucleicacidprobes. By varying the nucleicacidprobe, scientists candemonstratethe presence of different types of microorganisms.

The first scientists to use these techniques were involved in theSubsurfaceScience Program of the United StatesDepartmentof Energy (DOE). They were interested in the possibility that if organisms existed in the depths of Earth, they mightdegradeorganicpollutants and helpmaintainthepurityofgroundwateror, rather less usefully,degradethe containers in which the DOE was proposing todeposittheradioactivewastefrom nuclear facilities. They demonstrated the presence of many different types of microorganisms in rocks at depths down to 500 metersbeneaththe surface. Since then, microbes have been discovered in many different types of rocks and deepwithinocean sediments. Therecorddepth at which life has been found is at the bottom of a South African goldmine, 3.5 kilometers below ground. Pressureandtemperatureincrease as you go deeper into Earth. Some scientists think thatsubsurfacebacteriacouldwithstandtemperatures as high as 150℃. This would allow organisms to exist to depths of about 7 kilometersbeneaththe seafloor and to 4 kilometers below the surface of the land. Although the organisms are often sparsely distributed, this is such anenormousvolumethat it has been estimated that the total biomass of deepsubsurfaceorganisms exceeds that of those living on, or just below, the surface.

Bacteriaare the mostnumerousof thesesubsurfaceorganisms, but there are alsofungiand protozoa. Some 10,000 strains ofmicroorganismhave beenisolatedfromsubsurfacecores. Each gram of rock contains anything from 100bacteriato 10 millionbacteria(compared with more than 1 billion per gram in agricultural soils); ocean sedimentscontainevenhigher numbers. The protozoa feed on thebacteria, forming part of a simplesubterraneanfoodchain, but what do thebacteriafeed on?Sedimentaryrocks are formed from sands and from ocean, river, or lake sediments that haveorganicmaterialtrappedwithinthem. Microbes living in poreswithinthe sediments canutilizetheseancientnutrients and grow. Assedimentaryrocks are buried more deeply, they become increasingly compacted and their pores filled with minerals. Thedistributionof microorganisms is thus likely to become more patchy, condensed into theremainingpores and concentrations of nutrients. Thebulkof Earth'scrust, however, consists ofigneousrocks, such asgraniteand basalt, which are solidified frommoltenmagma. These rocks were too hot to support life when they were first formed; the organisms thatinhabitcracks and fissureswithinthe rocks are carried there by thegroundwaterflowingthroughthem. Subsurfacebacteriado not justrelyon nutrients trappedwithinthe rock or carried there bygroundwater. Some are chemotrophs, deriving their energy from the oxidation of iron orsulfurcompounds and buildingorganicmaterialdirectly from thecarbondioxideandhydrogengas dissolved in the rock. Thesebacteriaexcreteorganiccompounds that are then utilized by other types ofbacteria. These ecosystems based on chemotrophicbacteriaare completelyindependentofmaterialandsolarenergy from the surface.

Questions:
question_stem: The word "pristine" in the passage is closest in meaning to
choices: tag: A
text: suitable

tag: B
text: pure

tag: C
text: precise

tag: D
text: solid
answer_key: B
explanation: pristine adj.原本的，未受破坏的；与 pure 同义。
故选 B

question_stem: According to paragraph 1, what is the purpose of using concentrated tracer material during drilling?
choices: tag: A
text: To lubricate the drill

tag: B
text: To obtain uncontaminated rock samples

tag: C
text: To cut away the samples of rock

tag: D
text: To identify rock sample types
answer_key: B
explanation: 题干提问“钻井时使用示踪剂材料的目的是什么？”
根据 concentrated tracer material 定位第1段第6句，其中 any contaminated material could be identified and cut away to leave a pristine sample of rock 为其目的，即切去污染材料以便留下未受污染的样本，对应选项 B。
故选 B

question_stem: According to paragraph 2, what is the greatest depth at which life has been found to this day?
choices: tag: A
text: 500 meters below ground

tag: B
text: 3.5 kilometers below ground

tag: C
text: 4 kilometers below ground

tag: D
text: 7 kilometers beneath the seafloor
answer_key: B
explanation: 题干提问“至今已被发现的生物最深的深度？”
根据 has been found 定位第2段 The record depth at which life has been found is at the bottom of a South African gold mine, 3.5 kilometers below ground. 其中3.5 kilometers below ground 对应选项 B；
选项 C、D 虽然在文本中出现，也确实比3.5km 更深，但文中表达时只是猜想，并未证实，不符合题干的 has been found。
故选 B

question_stem: The author asks "what do the bacteria feed on?" for the purpose of
choices: tag: A
text: presenting a problem that scientists have not yet been able to solve

tag: B
text: expressing doubts about the theory that bacteria can live at great depths

tag: C
text: introducing the explanation that answers the question

tag: D
text: arguing against the idea that bacteria from part of a subterranean chain
answer_key: C
explanation: 题干提问“作者提及‘What do the bacteria feed on？’的目的是”。
根据 what do the bacteria feed on?定位第3段，作为设问句的功能是引起下文，其后第2句 Microbes living in pores within the sediments can utilize these ancient nutrients and grow. 确实在回答问题，符合设问句功能，对应选项 C。
故选 C

question_stem: The word "compacted" in the passage is closest in meaning to
choices: tag: A
text: complicated

tag: B
text: strengthened

tag: C
text: absorbent

tag: D
text: compressed
answer_key: D
explanation: compacted adj.压实的，坚实的；与 compressed 同义。
故选 D

question_stem: Which of the following can be inferred from paragraph 3 about the organisms in cracks and fissures in igneous rock?
choices: tag: A
text: They produce the cracks and fissures in the rock.

tag: B
text: They grow in the molten magma that forms the rock.

tag: C
text: They can survive changing temperature.

tag: D
text: They have not originated in the igneous rock.
answer_key: D
explanation: 题干提问“由第3段推断火成岩缝隙中的生物”。
根据 cracks and fissures in igneous rock 定位第3段 the organisms that inhabit cracks and fissures within the rocks are carried there by the groundwater flowing through them，其中 carried there by the groundwater flowing through them，即：被地下水带过来，而非原来就在此地，对应选项 D。
故选 D

question_stem: According to paragraph 3, why are chemotrophic bacteria important to their ecosystems?
choices: tag: A
text: They compete with other microorganisms.

tag: B
text: They allow other bacteria to use organic compounds.

tag: C
text: They do not depend on the iron or sulfur compounds that other microorganisms consume.

tag: D
text: They process material and solar energy for other underground organisms to use.
answer_key: B
explanation: 题干提问“为什么化学氧菌对其生态系统重要？”
根据 chemotrophic bacteria 定位第3段倒数第2句，These bacteria excrete organic compounds that are then utilized by other types of bacteria. 即：分泌有机化合物，然后被其他类型的细菌利用，对应选项 B。
选项 A 的 compete with 文中未提及，选项 C 的 do not depend on the iron or sulfur compounds 与原文说反，选项 D 的 process material and solar energy 与原文说反。
故选 B