B

There is increasing evidence that the impacts of meteorites (陨星) 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. 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 (白垩纪期) 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 (古生物学者) 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 (沉积物) deposited from the dust cloud that enveloped the planet after the impact. This sediment layer is enriched in the rare metal iridium (铱) and other elements that are relatively abundant in a meteorite but very rare in the crust (地壳) of Earth. Even diluted (稀释) by the terrestrial (地球的) material excavated (挖掘) 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. 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 (硝酸) 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 way 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.

5. In paragraph 2, why does the author include the information that dinosaurs had flourished for tens of millions of years and then suddenly disappeared?

A. To support the claim that the mass extinction at the end of the Cretaceous is the best-documented of the dozen or so mass extinctions in the geological record.

B. To explain why as many as half of the species on Earth at the time are believed to have become extinct at the end of the Cretaceous.

C. To explain why paleontologists have always been intrigued by the mass extinction at the end of the Cretaceous.

D. To provide evidence that an impact can be large enough to disturb the environment of the entire planet and cause an ecological disaster.

6. According to paragraph 3, how did scientists determine that a large meteorite had impacted Earth?

A. They discovered a large crater in the Yucat region of Mexico.

B. They found a unique layer of sediment worldwide.

C. They were alerted by archaeologists who had been excavating in the Yucat region.

D. They located a meteorite with a mass of over a trillion tons.

7. According to paragraph 4, all of the following statements are true of the impact at the end of the Cretaceous period EXCEPT:

A. A large amount of dust blocked sunlight from Earth.

B. Earth became cold and dark for several months.

C. New elements were formed in Earth's crust.

D. Large quantities of nitric acid were produced.

8. The phrase tentatively identified in the passage is closest in the meaning to ____________.

A. identified without certainty B. identified after careful study

C. occasionally identified D. easily identified

9. Paragraph 6 supports which of the following statements about the factors that are essential for the survival of a species?

A. The most important factor for the survival of a species is its ability to compete and adapt to gradual changes in its environment.

B. The ability of a species to compete and adapt to a gradually changing environment is not the only ability that is essential for survival.

C. Since most extinctions of species are due to major meteorite impacts, the ability to survive such impacts is the most important factor for the survival of a species.

D. The factors that are most important for the survival of a species vary significantly from one species to another.