The passage is primarily concerned with_出国考试答疑器_GMAT

Line	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
(5)	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
(10)	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
(15)	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:
(20)	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
(25)	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

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

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