The dry mountain ranges of the Western United States contain rocks dating back 440 to 510 million years, Line 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 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


Which of the following best describes the function of last sentence of the passage?


it points out that the events described in the passage may be atypical

it alludes to the fact that there is disagreement in the scientific community over the importance of the Ordovician radiation

it concludes that the evidence presented in the passage is insufficient to support the proposed hypothesis because it comes from a limited geographic area

it warns the reader against seeing a connection between the biological and geologic changes described in the passage

it alerts the reader that current knowledge cannot completely explain the relationship suggested by the evidence presented

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