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  3. The dry mountain ranges of the western United States contain rocks dating back 440 to 510 million years, to the Ordovician perio

The dry mountain ranges of the western United States contain rocks dating back 440 to 510 million years, to the Ordovician perio

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问题 The dry mountain ranges of the western United States contain rocks dating back 440 to 510 million years, to the Ordovician period, and teeming with evidence of tropical marine life. This rock record provides clues about one of the most significant radiations (periods when existing life-forms gave rise to variations that would eventually evolve into entirely new species) in the history of marine invertebrates. During this radiation the number of marine biological families increased greatly, and these families included species that would dominate the marine ecosystems of the area for the next 215 million years. Although the radiation spanned tens of millions of years, major changes in many species occurred during a geologically short time span within the radiation and, furthermore, appear to have occurred worldwide, suggesting that external events were major factors in the radiation. In fact, there is evidence of major ecological and geological changes during this period: the sea level dropped drastically and mountain ranges were formed. In this instance, rather than leading to large-scale extinctions, these kinds of environmental changes may have resulted in an enriched pattern of habitats and nutrients, which in turn gave rise to the Ordovician radiation. However, the actual relationship between these environmental factors and the diversification of life-forms is not yet fully understood.
The passage is primarily concerned with
选项 A、evaluating the evidence of a major geologic period and determining its duration
B、describing an evolutionary phenomenon and speculating about its cause
C、explaining the mechanisms through which marine life-forms evolved during a particular period
D、analyzing the impact on later life-forms of an important evolutionary development
E、contrasting a period of evolutionary change with other such periods
答案B
解析 Main idea
This question asks for an assessment of what the passage as a whole is doing. The passage is mainly concerned with a possible link between certain geological and ecological changes that occurred during the Ordovician period and the Ordovician radiation (when existing marine invertebrate life-forms gave rise to new variations that would eventually lead to new species).
A The passage is not particularly concerned with determining the length of the period in question.
B Correct. The passage is mainly concerned with a possible link between the evolutionary phenomenon of the Ordovician radiation and certain environmental changes that may have resulted in an enriched pattern of habitats and nutrients that could have fostered that radiation.
C The passage indicates that the particular mechanisms through which marine life-forms evolved are not well understood.
D Although the passage indicates that the changes it discusses ultimately did lead to new life-forms, it does not analyze that relationship.
E The passage does not discuss any period of evolutionary change besides the Ordovician radiation.
The correct answer is B.
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