When one considers the many ways by which organisms are completely destroyed after death, it isremarkablethat fossils are ascommonas they are. Attack by scavengers andbacteria, chemicaldecay, anddestructionbyerosionand other geologicagenciesmake the odds againstpreservationvery high. However, the chances of escaping completedestructionare vastly improved if theorganismhappens to have a mineralizedskeletonand dies in a place where it can be quickly buried bysediment. Both of these conditions are often found on the ocean floors, where shelled invertebrates (organisms without spines)flourishand are covered by thecontinuousrain ofsedimentaryparticles. Although most fossils are found inmarinesedimentaryrocks, they also are found interrestrialdeposits left by streams and lakes. On occasion, animals and plants have been preserved after becoming immersed in tar or quicksand, trapped in ice orlavaflows, or engulfed by rapid falls ofvolcanicash.

Theterm"fossil" often impliespetrifaction,literallyatransformationinto stone. After the death of anorganism, thesofttissueis ordinarily consumed by scavengers andbacteria. The emptyshellof a snail orclammay be left behind, and if it issufficientlydurableandresistantto dissolution, it mayremainbasically unchanged for a long period of time. Indeed, unaltered shells ofmarineinvertebrates are known from deposits over 100 million years old. In manymarinecreatures, however, theskeletonis composed of amineralvarietyofcalciumcarbonatecalled aragonite. Although aragonite has the samecompositionas the morefamiliarmineralknown as calcite, it has a differentcrystalform, isrelativelyunstable, and in time changes to the morestablecalcite.

Many other processes may alter theshellof aclamor snail andenhanceits chances forpreservation. Water containing dissolvedsilica,calciumcarbonate, or iron maycirculatethroughthe enclosingsedimentand be deposited in cavities such as marrow cavities and canals in bone once occupied by blood vessels and nerves. In such cases, theoriginalcompositionof the bone orshellremains, but thefossilis made harder and moredurable. Thisadditionof a chemically precipitatedsubstanceintoporespaces is termed "permineralization."

Petrifactionmay also involve asimultaneousexchangeof theoriginalsubstanceof a dead plant or animal withmineralmatter of a differentcomposition. Thisprocessis termed " replacement" because solutions have dissolved theoriginalmaterialand replaced it with anequalvolumeof the newsubstance. Replacement can be a marvelouslypreciseprocess, so that details ofshellornamentation, tree rings in wood, anddelicatestructures in bone are accurately preserved.

Another type offossilization, known as carbonization, occurs whensofttissues are preserved as thin films ofcarbon. Leaves andtissueof soft-bodied organisms such asjellyfishor worms mayaccumulate, become buried and compressed, and lose theirvolatileconstituents. Thecarbonoften remains behind as a blackened silhouette.

Although it is certainly true that thepossessionof hard parts enhances theprospectofpreservation, organisms havingsofttissues and organs are also occasionally preserved. Insects andevensmall invertebrates have been found preserved in the hardened resins of conifers and certain other trees. X-ray examination of thin slabs of rock sometimes reveals the ghostly outlines of tentacles,digestivetracts, andvisualorgans of avarietyofmarinecreatures. Softparts, including skin, hair, and viscera of ice age mammoths, have been preserved in frozen soil or in the oozing tar of oil seeps.

The probability that actual remains ofsofttissuewill be preserved is improved if theorganismdies in an environment of rapiddepositionandoxygendeprivation. Under such conditions, thedestructiveeffects ofbacteriaarediminished. The Middle Eocene MesselShale(from about 48 million years ago) of Germany accumulated in such an environment. Theshalewas deposited in an oxygen-deficient lake wherelethalgases sometimes bubbled up and killed animals. Their remains accumulated on the floor of the lake and were then covered byclayandsilt. Among the superbly preserved Messel fossils are insects with iridescent exoskeletons (hard outer coverings), frogs with skin and blood vesselsintact, andevenentire small mammals with preservedfurandsofttissue.

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

tag: B
text: problems

tag: C
text: forces

tag: D
text: changes
answer_key: C
explanation: C agency: 力量，代理。A 选项是结合；B 是问题；C 是力量；D 是改变。所以答案是 C 的 force。

question_stem: In paragraph 1, what is the author's purpose in providing examples of how organisms are destroyed?
choices: tag: A
text: To emphasize how surprising it is that so many fossils exist

tag: B
text: To introduce a new geologic theory of fossil preservation

tag: C
text: To explain why the fossil record until now has remained incomplete

tag: D
text: To compare how fossils form on land and in water
answer_key: A
explanation: A 修辞目的题，修辞点所在句是一个例子，所以往前看，前一句说 fossil 遭破坏的方式和 fossil 一样多，后面就跟了很多破坏的方式，所以答案是 A，阐释为什么如此多破坏之下还有这么多化石存在。往后看也可以，下一句说如果化石有骨架的话被保留的机会会大增，也就是一直都在说化石存留下来的机会，所以 A 是正确答案

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

tag: B
text: protected

tag: C
text: alternative

tag: D
text: similar
answer_key: A
explanation: A, terrestrial: 陆地的，陆生的。A 是陆地的；B 是受保护的；C 是供选择的；D 是相似的。所以正确答案是 A。

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: When snail or clam shells are left behind, they must be empty in order to remain durable and resist dissolution.

tag: B
text: Although snail and clam shells are durable and resist dissolving, over time they slowly begin to change.

tag: C
text: Although the soft parts of snails or clams dissolve quickly, their hard shells resist dissolution for a long time.

tag: D
text: Empty snail or clam shells that are strong enough not to dissolve may stay in their original state for a long time.
answer_key: D
explanation: D 原句的结构是并列加条件，并列的部分是：1.shell/clam 会被留下；2）保持不变。条件逻辑中条件是“对分解有足够持久的抵抗”。所以正确答案是 D。A 的 must be empty 原文没说；B 和 C 的逻辑关系都错误，原句没有转折；D 说的是 shell 会被剩下，如果足够耐腐蚀，就能保存一段时间，正确。

question_stem: Why does the author mention "aragonite" in the passage?
choices: tag: A
text: To emphasize that some fossils remain unaltered for millions of years

tag: B
text: To contrast fossil formation in organisms with soft tissue and in organisms with hard shells

tag: C
text: To explain that some marine organisms must undergo chemical changes in order to fossilize

tag: D
text: To explain why fossil shells are more likely to survive than are fossil skeletons
answer_key: C
explanation: C 修辞目的题，先看例子所在句子，说很多海洋生物的骨骼包含 calcite，没有答案，往前看，前一句说一亿年前的沉积物中能发现骨骼不变的海生无脊椎动物，与 A 对对应，但例子本身所说的 aragonite 的组成成分不怎么稳定，所以例子并不是用来支撑前一句话的，A 不对；强调的中心在例子所在句的下一句，说 arogonite 的晶体形状不同，相对不那么稳定，会变成更稳定的形式，所以答案是 C，想稳定的话必须再变。B 和 D 完全没说，不对。

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

tag: B
text: limit

tag: C
text: combine

tag: D
text: increase
answer_key: D
explanation: D enhance: 提升，提高，增加。A 是控制；B 是限制；C 是结合；D 是增加，提高。所以正确答案是 D 的 increase。

question_stem: Which of the following best explains the process of permineralization mentioned in paragraph 3?
choices: tag: A
text: Water containing calcium carbonate circulates through a shell and deposits sediment.

tag: B
text: Liquid containing chemicals hardens an already existing fossil structure.

tag: C
text: Water passes through sediment surrounding a fossil and removes its chemical content.

tag: D
text: A chemical substance enters a fossil and changes its shape.
answer_key: B
explanation: B 以 permineralization 做关键词定位至最后一句。说将 chemically precipitate 的物质加入 precipitate 的过程叫做 permineralization，this 指代前文，所以往前看，前面说：血管和神经占据的空隙会被含有各种物质的 water 充填，骨头和壳体会留下来，变得更坚固，所以答案是 B，变得更坚硬。A 的包含 calcium 不全面，而且原文的变硬也没说；C 的 remove chemical content 和 D 的 change shape 都没说。

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

tag: B
text: quick

tag: C
text: exact

tag: D
text: reliable
answer_key: C
explanation: C precise: 精确的。A 是复杂的；B 是快速的；C 是精确地；D 是可靠的。所以 C 的 exact 正确。

question_stem: Paragraph 5 suggests which of the following about the carbonization process?
choices: tag: A
text: It is completed soon after an organism dies.

tag: B
text: It does not occur in hard-shell organisms.

tag: C
text: It sometimes allows soft-tissued organisms to be preserved with all their parts.

tag: D
text: It is a more precise process of preservation than is replacement.
answer_key: B
explanation: B 此段较短，完全可以快速扫完。A 错，原文说生命死后要经过若干过程才会 carbonization，所以 A 的 soon 明显错；B 正确，原文明确指出 carbonization 发生在柔软组织中，当然也就不会发生在硬壳动物身上；C 的 all their parts 和 D 的比较原文都没说，都错。

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

tag: B
text: variety

tag: C
text: possibility

tag: D
text: speed
answer_key: C
explanation: C prospect: 可能性，希望。改词也有前景的意思，但原文是可能性的意思。A 是完成；B 是多样（化）；C 是可能性；D 是速度。所以 C 的 possibility 正确。

question_stem: According to paragraph 7, how do environments containing oxygen affect fossil preservation?
choices: tag: A
text: They increase the probability that soft-tissued organisms will become fossils.

tag: B
text: They lead to more bacteria production.

tag: C
text: They slow the rate at which clay and silt are deposited.

tag: D
text: They reduce the chance that animal remains will be preserved.
answer_key: D
explanation: D 以 oxygen 做关键词定位至第一句，说如果快速埋藏，并且在缺氧环境下，化石保存的概率会增加，也就是说氧的存在不利于化石的保存，所以正确答案是 D，A 与 D 刚好完全相反，所以 A 错，也说明两个相反的有一个对；C 完全没说；B 错，原文说缺氧能减轻细菌的破坏作用，但并不意味着有氧细菌就会变多，所以 B 没说。

question_stem: According to the passage, all of the following assist in fossil preservation EXCEPT
choices: tag: A
text: the presence of calcite in an organism's skeleton

tag: B
text: the presence of large open areas along an ocean floor

tag: C
text: the deposition of a fossil in sticky substances such as sap or tar

tag: D
text: the rapid burial of an organism under layers of silt
answer_key: B
explanation: B，EXCEPT 题，排除法，由于考全文，应该关注各段开头。A 的 skeleton 和 calcite 做关键词定位至第六段首句，正确，不选；B 的 ocean floor 在原文中虽然有出现，但 open areas 没有，所以 B 错，选；C 的 sap or tar 做关键词定位至第六段最后一句，正确，不选；D 的 rapid burial 和 layer of silt 做关键词定位至第七段第一句，正确，不选。