On the basis of the available evidence, Antarctica has generally been thought to have been covered by ice for at least the past 14 million years. Recently, however, three-million-year-old fossils of a kind previously found only in ocean-floor sediments were discovered under the ice sheet covering central Antarctica. About three million years could have melted the ice sheet, thus raising sea levels and submerging the continent.
What this question is testing
Conclusion
The author wants you to walk away believing this: the Antarctic ice sheet, which scientists thought had been there for 14 million years straight, must have temporarily melted around three million years ago.
Evidence
The author has two pieces of support: (1) fossils typically found in ocean-floor sediments turned up under the ice — meaning at some point that area was ocean, not ice; and (2) either volcanic activity or warming would have been physically capable of causing such a melt.
Evaluate
The trick on Main Conclusion questions: read for "therefore," "thus," and the phrase that follows "after all" (which signals support, not conclusion). Here, "therefore, the ice sheet must temporarily have melted" is what the author wants you to believe. Everything else is either background ("scientists used to think...") or support ().
That main conclusion can be restated as: the ice has not been continuously present for the past 14 million years — because it melted at the three-million-year mark.
Goal
The answer should match: the ice sheet has not been continuously present throughout the past 14 million years.
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