Knowledge of Earth's deepinterioris derived from the study of the waves produced by earthquakes, calledseismicwaves. Among the various kinds ofseismicwaves areprimarywaves(P-waves) and secondary waves(S-waves). Primaryand secondary waves pass deepwithinEarth andthereforeare the mostinstructive. Study ofabruptchanges in the characteristics ofseismicwaves at different depths provides the basis for a threefolddivisionof Earth into a centralcore; a thick, overlyingmantle; and a thin, envelopingcrust. Sudden changes inseismicwave velocities and angles of transmission are termed discontinuities.

One of the discontinuities is the Gutenberg discontinuity, which is located nearly halfway to the center of Earth at a depth of 2,900 kilometers and marks the outerboundaryof Earth'score. At that depth, the S-waves cannotpropagate, while at the same time P-wavevelocityisdrasticallyreduced. S-waves are unable to travelthroughfluids. Thus, if S-waves were toencounterafluidlikeregionof Earth'sinterior, they would be absorbed there and would not be able to continue. Geophysicists believe this is what happens to S-waves as they enter the outercore. As a result, the S-waves generated on one side of Earth fail toappearatseismographstations on theoppositeside of Earth, and thisobservationis theprincipalevidenceof an outercorethat behaves as afluid. Unlike S-waves, P-waves are able to passthroughliquids. They are, however,abruptlyslowed andsharplyrefracted(bent) as they enter afluidmedium. Therefore, as P-wavesencounterthemoltenoutercoreof Earth, theirvelocityis reduced and they are refracteddownward.

Theradiusof thecoreis about 3,500 kilometers. Theinnercoreissolidand has aradiusof about 1,220 kilometers, which makes thisinnercoreslightlylarger than the Moon. Most geologists believe that theinnercorehas the samecompositionas the outercoreand that it can only exist as asolidbecause of theenormouspressureat the center of Earth. Evidenceof theexistenceof asolidinnercoreis derived from the study of hundreds of records ofseismicwaves produced over several years. These studies showed that theinnercorebehaves seismically as if it were asolid.

Earth has anoveralldensityof 5.5 grams per cubic centimeter, yet theaveragedensityof rocks at the surface is less than 3.0 grams per cubic centimeter. Thisdifferenceindicates that materials of highdensitymust exist in the deepinteriorof theplanetto achieve the 5.5 grams per cubic centimeteroveralldensity. Under theextremepressureconditions that exist in theregionof thecore, iron mixed withnickelwould very likely have the required highdensity. Laboratoryexperiments, however, suggest that a highly pressurized iron-nickelalloymight be toodenseand thatminoramounts of such elements such assilicon,sulfur,carbon, oroxygenmay also be present to lighten thecorematerial.

Support for thetheorythat thecoreis composed of iron (85 percent) with lesser amounts ofnickelhas come from the study of meteorites. Many of these samples ofsolarsystem materials are iron meteorites thatconsistofmetalliciron alloyed with a small percentage ofnickel. Some geologistssuspectthat iron meteorites may be fragments from thecoreof a shatteredplanet. The presence of iron meteorites in oursolarsystem suggests that theexistenceof an iron-nickelcorefor Earth isplausible.

There is furtherevidencethat Earth may have ametalliccore. Anyone who understands the functioning of acompassisawarethat Earth has amagneticfield. Theplanetitself behaves as if there was a great barmagnetembeddedwithinit. Amagneticfieldis developed by the flow of electric charges and requires good electrical conductors. Silicaterocks, such as those in themantleandcrust, do notconductelectricityvery well, whereas metals such as iron andnickelare good conductors. Heat-driven movements in the outercore, coupled with movements induced by Earth'sspin, are thought to provide the necessary flow of electrons (very small particles that carry anegativecharge) around theinnercorethat produces themagneticfield. Without ametalliccore,Earth'smagneticfieldwould not be possible.

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

tag: B
text: intense

tag: C
text: inaccessible

tag: D
text: informative
answer_key: D
explanation: instructive adj.富有教益的，增长知识的；与 informative 同义。
故选 D

question_stem: According to paragraph 1, which of the following findings provides the basis for Earth's division into three parts?
choices: tag: A
text: All seismic waves pass deep within Earth.

tag: B
text: There are no significant differences in the behavior of P-waves and S-waves.

tag: C
text: Discontinuities occur at different depths within Earth.

tag: D
text: Earth's crust is too thin to account for the size of the planet.
answer_key: C
explanation: 题干提问“地球分为三部分的依据是”。
根据 basis 定位第1段后2句，Study of abrupt changes in the characteristics of seismic waves at different depths … … Sudden changes are termed discontinuities. 归纳总结可对应选项 C。
故选 C

question_stem: Paragraph 3 supports which of the following statements about the inner core?
choices: tag: A
text: It is smaller in volume than the Moon.

tag: B
text: It consists of materials different from those found in the outer core.

tag: C
text: It is subject to more pressure than the outer core.

tag: D
text: It is not solid even though it behaves seismically as if were.
answer_key: C
explanation: 题干提问“第3段支持有关内核的以下哪个陈述”。
定位第3段第3句 it can only exist as a solid because of the enormous pressure at the center of Earth 对应选项 C；
选项 A 中 smaller 与第2句 inner core slightly larger than the Moon 说反，错误；选项 B 中 different 与第3句 the inner core has the same composition as the outer core 说反，错误；选项 D 中 not solid 与最后一句 the inner core behaves seismically as if it were a solid 说反，错误。
故选 C

question_stem: According to paragraph 4, why do scientists believe that Earth's core consists of materials of very high density?
choices: tag: A
text: Under the extreme pressure conditions near the center of Earth, any materials would become very dense.

tag: B
text: Without a very dense core, Earth's overall density would not be as high as it is found to be.

tag: C
text: Earth's deep interior is known to be solid, and solids are always denser than liquids.

tag: D
text: Earth's core is known to contain the same high-density rocks that are formed on Earth's surface.
answer_key: B
explanation: 题干提问“为什么科学家相信地核由非常高密度的物质组成？”
根据 high density 定位第4段前2句，理解为地球密度5.5，岩石密度小于3，一定要有更高密度的物质与岩石综合才能实现5.5的密度，对应选项 B。
故选 B

question_stem: According to paragraph 4, why do scientists believe that Earth's core contains materials other than iron mixed with nickel?
choices: tag: A
text: In the absence of such materials, the core might be too dense.

tag: B
text: Iron and nickel do not mix evenly without the presence of lighter elements.

tag: C
text: Laboratory experiments showed that minor amounts of silicon, sulfur, carbon and oxygen are present in samples from Earth's core.

tag: D
text: Iron-nickel alloys always contain minor amounts of elements such as silicon, sulfur, carbon, or oxygen.
answer_key: A
explanation: 题干提问“为什么科学家相信地核包含除铁和镍的其他物质？”
定位第4段最后一句，a highly pressurized iron-nickel alloy might be too dense and that minor amounts of such elements such as silicon, sulfur, carbon, or oxygen may also be present to lighten the core material，即：高压下的铁镍合金密度过高，可能需要其他物质综合进而降低密度，对应选项 A。
故选 A

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

tag: B
text: revolving

tag: C
text: formed

tag: D
text: enclosed
answer_key: D
explanation: embedded adj.嵌入式的，内含的；与 enclosed 同义。
故选 D

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: A combination of different motions produces Earth's magnetic field, and the magnetic field is thought to cause a flow of electrons around the planet's inner and outer core.

tag: B
text: The flow of electrons around the planet that produces Earth's magnetic field is the combined effect of motions that are thought to occur in the planet's inner core.

tag: C
text: The flow of electrons around the inner core that produces Earth's magnetic field is thought to be the joint result of motions in the outer core and motions caused by Earth's spin.

tag: D
text: Heat-driven motions in the outer core interact with motions caused by the spin of the inner core to produce a flow of electrons that is thought to result in Earth's magnetic field.
answer_key: C
explanation: 定位第6段简化句，分析：Heat-driven movements in the outer core 与 movements induced by Earth's spin 一起提供了 the necessary flow of electrons around the inner core，从而产生了 the magnetic field。
选项 A 的 the magnetic field is thought to cause a flow of electrons 因果关系与原文说反，选项 B 的 around the planet 修饰错误，选项 D 的 the inner core 修饰错误；
选项 C 符合原文因果 The flow of electrons around the inner core that produces Earth's magnetic field，同时符合并列的两个作用力 joint result of motions in the outer core and motions caused by Earth's spin。
故选 C

question_stem: In paragraph 6, why does the author discuss Earth's magnetic field?
choices: tag: A
text: To cast doubt on its existence, given what scientists now know about Earth's composition

tag: B
text: To provide additional support for the theory of an iron-nickel core of Earth

tag: C
text: To present a puzzling feature of Earth that has not yet been explained satisfactorily

tag: D
text: To explain why so many iron meteorites are found near Earth
answer_key: B
explanation: 题干提问“作者讨论地球磁场的目的是”。
第6段从第2句开始到结尾一直在讨论地球磁场，作为论据佐证第1句观点 There is further evidence that Earth may have a metallic core. 对应选项 B。
故选 B