请教黄金真题里面的23的最后主旨题，怀疑答案有误

sumerlaw

Meteorite Impact and Dinosaur Extinction
There is increasing evidence that the impacts of meteorites have had important effects on Earth, particularly in the field of biological evolution. Such impacts continue to pose a natural hazard to life on Earth. Twice in the twentieth century, large meteorite objects are known to have collided with Earth.



If an impact is large enough, it can disturb the environment of the entire Earth and cause an ecological catastrophe. The best-documented such impact took place 65 million years ago at the end of the Cretaceous period of geological history. This break in Earth's history is marked by a mass extinction, when as many as half the species on the planet became extinct. While there are a dozen or more mass extinctions in the geological record, the Cretaceous mass extinction has always intrigued paleontologists because it marks the end of the age of the dinosaurs. For tens of millions of years, those great creatures had flourished. Then, suddenly, they disappeared.



The body that impacted Earth at the end of the Cretaceous period was a meteorite with a mass of more than a trillion tons and a diameter of at least 10 kilometers. Scientists first identified this impact in 1980 from the worldwide layer of sediment deposited from the dust cloud that enveloped the planet after the impact. This sediment layer is enriched in the rare metal iridium and other elements that are relatively abundant in a meteorite but very rare in the crust of Earth. Even diluted by the terrestrial material excavated from the crater, this component of meteorites is easily identified. By 1990 geologists had located the impact site itself in the Yucat region of Mexico
. The crater, now deeply buried in sediment, was originally about 200 kilometers in diameter.


This impact released an enormous amount of energy, excavating a crater about twice as large as the lunar crater Tycho. The explosion lifted about 100 trillion tons of dust into the atmosphere, as can be determined by measuring the thickness of the sediment layer formed when this dust settled to the surface. Such a quantity of material would have blocked the sunlight completely from reaching the surface, plunging Earth into a period of cold and darkness that lasted at least several months. The explosion is also calculated to have produced vast quantities of nitric acid and melted rock that sprayed out over much of Earth, starting widespread fires that must have consumed most terrestrial forests and grassland. Presumably, those environmental disasters could have been responsible for the mass extinction, including the death of the dinosaurs.



Several other mass extinctions in the geological record have been tentatively identified with large impacts, but none is so dramatic as the Cretaceous event. But even without such specific documentation, it is clear that impacts of this size do occur and that their results can be catastrophic. What is a catastrophe for one group of living things, however, may create opportunities for another group. Following each mass extinction, there is a sudden evolutionary burst as new species develop to fill the ecological niches opened by the event.



Impacts by meteorites represent one mechanism that could cause global catastrophes and seriously influence the evolution of life all over the planet. According to some estimates, the majority of all extinctions of species may be due to such impacts. Such a perspective fundamentally changes our view of biological evolution. The standard criterion for the survival of a species is its success in competing with other species and adapting to slowly changing environments. Yet an equally important criterion is the ability of a species to survive random global ecological catastrophes due to impacts.



Earth is a target in a cosmic shooting gallery, subject to random violent events that were unsuspected a few decades ago. In 1991 the United States Congress asked NASA to investigate the hazard posed today by large impacts on Earth. The group conducting the study concluded from a detailed analysis that impacts from meteorites can indeed be hazardous. Although there is always some risk that a large impact could occur, careful study shows that this risk is quite small.

14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.



Scientists have linked the mass extinction at the end of the Cretaceous with a meteorite impact on Earth.（怀疑答案不对，2明显是细节，1是主要的）

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Answer choices

○Scientists had believed for centuries that meteorite activity influenced evolution on Earth.

○An iridium-enriched sediment layer and a large impact crater in the Yucat provide evidence that a large meteorite struck Earth about 65 million years ago.

○The site of the large meteorite impact at the end of the Cretaceous period was identified in 1990.

○Large meteorite impacts, such as one at the end of the Cretaceous period, can seriously affect climate, ecological niches, plants, and animals.

○There have also been large meteorite impacts on the surface of the Moon, leaving craters like Tycho.

○Meteorite impacts can be advantageous for some species, which thrive, and disastrous for other species, which become extinct.

请教高人！


14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.



Scientists have linked the mass extinction at the end of the Cretaceous with a meteorite impact on Earth.（怀疑答案不对，2明显是细节，1是主要的）

●

●

●

Answer choices

○Scientists had believed for centuries that meteorite activity influenced evolution on Earth.

○An iridium-enriched sediment layer and a large impact crater in the Yucat provide evidence that a large meteorite struck Earth about 65 million years ago.

○The site of the large meteorite impact at the end of the Cretaceous period was identified in 1990.

○Large meteorite impacts, such as one at the end of the Cretaceous period, can seriously affect climate, ecological niches, plants, and animals.

○There have also been large meteorite impacts on the surface of the Moon, leaving craters like Tycho.

○Meteorite impacts can be advantageous for some species, which thrive, and disastrous for other species, which become extinct.

请教高人！

sumerlaw

等NN 来帮忙

Donna082

这道题我也做错了  不过后来如果从联系的角度来说似乎2更符合要求 1中的for centries是不是不太恰当 题目要求是灭绝和对地球影响的关系   还请高人指点

sumerlaw

这道题我也做错了  不过后来如果从联系的角度来说似乎2更符合要求 1中的for centries是不是不太恰当 题目要求是灭绝和对地球影响的关系   还请高人指点

-- by 会员 Donna082 (2010/3/9 12:19:42)

谢谢Donna082的点拨，
我明白了，1中的for centuries和原文的unsuspected a few decades ago相矛盾
而2的话则表明了确定恐龙灭绝前发生碰撞的时间，是重要信息，因为文章主要讨论的是恐龙灭绝和流行的影响，2 都包括了。
ps:建议斑竹能不能整个讨论贴方便大家学习交流