Cases in which manyspeciesbecomeextinctwithin a geologically short interval of time are calledmassextinctions. There was one such event at the end of theCretaceousperiod (around 70 million years ago). There was another,evenlarger,massextinctionat the end of the Permian period (around 250 million years ago). The Permian event has attracted much lessattentionthan othermassextinctions because mostly unfamiliarspeciesperished at that time.

Thefossilrecordshows atleastfivemassextinctions in which many families ofmarineorganisms died out. The rates ofextinctionhappening today are as great as the rates during thesemassextinctions. Many scientists havethereforeconcluded that a sixth greatmassextinctionis currently in progress.

What could cause such high rates ofextinction? There are several hypotheses, including warming or cooling of Earth, changes inseasonalfluctuations or ocean currents, and changing positions of the continents. Biological hypotheses includeecologicalchanges brought about by theevolutionofcooperationbetween insects and flowering plants or of bottom-feeding predators in the oceans. Some of the proposed mechanisms required a verybriefperiod during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place moregradually, over an extended period, or at different times on different continents. Some hypotheses fail toaccountforsimultaneousextinctions on land and in the seas. Eachmassextinctionmay have had a different cause. Evidencepoints to hunting by humans andhabitatdestructionas the likely causes for thecurrentmassextinction.

American paleontologists David Raup and John Sepkoski, who have studiedextinctionrates in a number offossilgroups, suggest that episodes of increasedextinctionhave recurredperiodically,approximatelyevery 26 million years since the mid-Cretaceous period. The lateCretaceousextinctionof the dinosaurs and ammonoids was just one of the moredrasticin a wholeseriesof such recurrentextinctionepisodes. The possibility thatmassextinctions mayrecurperiodicallyhasgivenrise to such hypotheses as that of acompanionstar with a long-periodorbitdeflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causingwidespreaddevastation uponimpact.

Of the various hypotheses attempting toaccountfor the lateCretaceousextinctions, the one that has attracted the mostattentionin recent years is the asteroid-impacthypothesisfirst suggested by Luis and Walter Alvarez. According to thishypothesis, Earth collided with anasteroidwith an estimateddiameterof 10 kilometers, or with several asteroids, the combinedmassof which wascomparable. The force ofcollisionspewed large amounts ofdebrisinto theatmosphere, darkening the skies for several years before the finer particles settled. The reducedlevelofphotosynthesisled to amassivedeclinein plant life of all kinds, and this causedmassivestarvationfirst of herbivores andsubsequentlyof carnivores. Themassextinctionwould have occurred very suddenly under thishypothesis.

One interesting test of the Alvarezhypothesisis based on the presence of the rare-earthelementiridium(Ir). Earth’scrustcontains very little of thiselement, but most asteroidscontaina lot more. Debristhrown into theatmosphereby anasteroidcollisionwouldpresumablycontainlarge amounts ofiridium, andatmosphericcurrents would carry thismaterialall over the globe. A search ofsedimentarydeposits thatspantheboundarybetween theCretaceousandTertiaryperiods shows that there is adramaticincrease in theabundanceofiridiumbrieflyand precisely at thisboundary. Thisiridiumanomalyoffers strong support for the Alvarezhypothesiseventhough noasteroiditself has ever been recovered.

Anasteroidof this size would be expected to leave animmensecrater,evenif theasteroiditself was disintegrated by theimpact. Theintenseheat of theimpactwould produce heat-shockedquartzin many types of rock. Also, large blocks thrown aside by theimpactwould form secondary craters surrounding the maincrater. Todate, several such secondary craters have been foundalongMexico’s Yucatan Peninsula, and heat-shockedquartzhas been found both in Mexico and in Haiti. A location called Chicxulub,alongthe Yucatancoast, has been suggested as theprimaryimpactsite.

Questions:
question_stem: Paragraph 1 supports which of the following statements about mass extinctions?
choices: tag: A
text: They take place over a period of 70 million years.

tag: B
text: They began during the Cretaceous period.

tag: C
text: They eliminate many animal species that exist at the time they occur.

tag: D
text: They occur every 250 million years.
answer_key: C
explanation: C 以 mass extinctions 做关键词定位至第一句,说大量生物在短时间内灭绝的这种现象叫做大灭绝事件，C 是原文的同义替换，所以是正确答案。70 million 和250 million 是两次大灭绝事件发生的时间，不是 A 说的七千万年一直在发生，也不是 D 说的每2500万年发生一次；B 的 began 和原文的 end 是反的，错。

question_stem: According to paragraph 2, scientists base their belief that a mass extinction is going on at present on which of the following?
choices: tag: A
text: The speed with which mass extinctions are happening today is similar to the speed of past extinctions.

tag: B
text: The number of species that have died out since the last extinction event is extremely large.

tag: C
text: Mass extinctions occur with regularity and it is time for another one.

tag: D
text: Fossil records of many marine species have disappeared.
answer_key: A
explanation: A 以 at present 做关键词定位至最后一句，但这段很短，快速扫完后两句之后就有答案，说现代生物灭绝的速度和大灭绝的时候差不多，得出大灭绝现在正在发生，所以原因是灭绝速度相似，答案 A。

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

tag: B
text: unlimited

tag: C
text: reasonable

tag: D
text: long
answer_key: D
explanation: extend 延伸，延长，extended 长期的，所以 long 是正确答案。原句说其他机制可能会渐渐发生，在一个什么样的时期内，既然是渐渐发生，当然需要比较长的时间，所以答案是 D。虽然长但不是无限，所以 unlimited 错；specific 特定时间和 reasonable 合理时间原文都没说。

question_stem: According to paragraph 3, each of the following has been proposed as a possible cause of mass extinctions EXCEPT
choices: tag: A
text: habitat destruction

tag: B
text: continental movement

tag: C
text: fierce interspecies competition

tag: D
text: changes in Earth's temperature
answer_key: C
explanation: 本段第一句就问了原因第二句就开始回答，所以这道题正选比较好。第二句的 warming or cooling 对应 D 答案，正确，不选；changing position 对应 B 答案，正确，不选；最后一句的 habitat destruction 对应 A 答案，正确，不选；只有 C 没有对应，所以 C 错，选。

question_stem: Paragraph 3 supports which of the following ideas about mass extinctions?
choices: tag: A
text: Scientists know the exact causes of most mass extinctions.

tag: B
text: Mass extinctions are unlikely to happen again in the future.

tag: C
text: Insects, flowering plants, and bottom-feeding predators in the oceans tend to be the first organisms to disappear during episodes of mass extinctions.

tag: D
text: Some mass extinctions occurred on land and in the seas at the same time.
answer_key: D
explanation: 问全段的，最好用排除法。A 的 exact causes 做关键词定位至前两句，原文说有很多假设，所以没有准确原因，A 错；B 的 future 原文没说，所以错；C 的一大堆生物做关键词定位至第三句，但原文没说它们是最先遭殃的一群生物，C 错；D 的 land and seas 做关键词定位至倒数第三句，simultaneously 就是 at the same time，正确.

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: Based on their studies of extinction rates of numerous fossil groups, paleontologists David Raup and John Sepkoski have determined that mass extinctions occur about every 26 million years.

tag: B
text: David Raup and John Sepkoski studied extinction rates of numerous fossil groups and suggest that mass extinctions during the Cretaceous period continued for 26 million years.

tag: C
text: Studies that paleontologists David Raup and John Sepkoski conducted of various fossil groups have revealed that extinction rates have increased over the past 26 million years.

tag: D
text: The studies conducted by paleontologists David Raup and John Sepkoski of the fossil remains of species suggest that the extinction rate of species started to increase by the middle of the Cretaceous period.
answer_key: A
explanation: 原句说这两个人认为 mass extinction 是重复出现的。正确答案是 A，C 和 D 的错误很容易分辨，因为 C 和 D 的谓语动词 increase 是原文没说到的，所以 C 和 D 都错。B 项颇具迷惑性，但 B 错在将 suggest 和 study 两个动词并列在一起，但原句是先 study，之后才 suggest，虽然没有明确指出 A 的 base on，但先有研究后有结论，这个结论当然与研究有关，所以 A 正确。

question_stem: According to paragraph 4, what aspect of extinction episodes does the companion-star hypothesis supposedly clarify?
choices: tag: A
text: Their location

tag: B
text: Their frequency

tag: C
text: Their duration

tag: D
text: Their severity
answer_key: B
explanation: 以 companion star hypothesis 做关键词定位至最后一句，说 mass extinction 的反复发生支持了 companion star hypothesis，这个 hypothesis 说轨道变形导致一些星体偏离正常轨道，成为陨星掉入地球，原句说屡次发生，所以能够解释 extinction 的频率，答案是 B.

question_stem: The phrase "account for" in the passage is closest in meaning to
choices: tag: A
text: describe

tag: B
text: challenge

tag: C
text: explain

tag: D
text: test
answer_key: C
explanation: account for 负责，解释，所以正确答案是 explain。原句说在所有试图什么晚白垩世生物大灭绝事件的假说当中，陨星撞击说是最吸引人的。运行撞击能够怎么样生物灭绝，A 的 challenge 和 D 的 test 意思相似，二者是解释关系，不是反驳或者测试彼此，所以两个答案都不对，而陨星撞击更不是简单描述晚白垩世生物灭绝，所以 describe 不对

question_stem: According to paragraph 6, what made iridium a useful test of the Alvarez hypothesis?
choices: tag: A
text: Its occurrence in a few locations on Earth against several locations on other planets

tag: B
text: Its occurrence in limited quantities on Earth against its abundance in asteroids

tag: C
text: Its ability to remain solid at extremely high temperatures

tag: D
text: Its ease of detection even in very small amounts
answer_key: B
explanation: 以人名做关键词定位至第一句，但第一句没说为什么铱测试对验证 A 的假说很有用，往下看，说地球上 Ir 几乎没有，但其他星体含量多很多，同义改写的答案是 B。也就是说如果地球上发现大量 Ir，就说明遭受过撞击。

question_stem: In stating that "no asteroid itself has ever been recovered", the author emphasizes which of the following?
choices: tag: A
text: The importance of the indirect evidence for a large asteroid

tag: B
text: The fact that no evidence supports the asteroid impact hypothesis

tag: C
text: The reason many researchers reject the Alvarez hypothesis

tag: D
text: The responsibility of scientists for not making the effort to discover the asteroid itself
answer_key: A
explanation: 修辞目的题，看本句先，原句说 Ir 异常支持了撞击说，但还没发现具体的星体。所以 B 说没有 evidence 错，说反了；C 有人 reject 他的观点，还有 D 的 scientists 的责任原文都说，不选；正确答案是 A，只有 Ir 异常这个间接证据就可以支持撞击说，所以是 indirect evidence

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

tag: B
text: unusual

tag: C
text: immediate

tag: D
text: extreme
answer_key: D
explanation: intense 高强度的，密集的，极端的，所以 D 的 extreme 正确。原句说撞击产生的什么样的热量可以产生出很多的 heat-shocked 石英，都 heat shocked 了，肯定是 heat 很大很多，而 sudden 和 immediate 都表示时间概念不表示多少所以都错；B 的 unusual 虽然可能表示多，但不一定，也有可能表示时间概念，错。

question_stem: What is the purpose of paragraph 7 in the passage?
choices: tag: A
text: It proposes a decisive new test of the Alvarez hypothesis.

tag: B
text: It presents additional supporting evidence for the Alvarez hypothesis.

tag: C
text: It explains why evidence relating to the Alvarez hypothesis is hard to find.

tag: D
text: It shows how recent evidence has raised doubts about the Alvarez hypothesis.
answer_key: B
explanation: 问整段的题关注头尾，开头说撞击会留下大坑，又说热量 blabla，结尾说某某地方被认为是撞击发生的地方，所以应该是支持撞击理论，所以 B 的 additional supporting evidence 正确；D 的 doubts 不对；C 说撞击位置很难找到与最后一句相反，错；A 的 test 原文没说。