求助 GWD TN22 4

victoria_cc

Q4~Q6: GWD32—32-34
    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. And , 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 pattem 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

Q4: The passage is primarily concerned with
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
正确答案为C  可我选的B

请nn指教这篇文章的主旨是怎样体现在解释mechanisms的
行文的结构脉络

谢谢～