为了让我们更好的适应sat考试，sat官网每天都会有sat官方每日一题，下面是小编为您整理的2016年4月8日SAT官方每日一题。
Critical Reading>Select an Answer
Four billion years ago, Earth's continentsbegan taking shape. Scientists long heldthat this process unfolded gradually. Itwould have taken millions of years for granite, the primary component ofthe continental crust, to form in the mantle and migrate to the upper crust,they reasoned. But new research suggests that these events may haveproceeded at rather a different pace. According to a report published in thejournal Nature, the emergence of granite occurred by way of swift,dynamic and possibly catastrophic events lasting from 1,000 to 100,000years.
Geologist Alexander Cruden of the University of Toronto and his colleaguesturned to experimental studies—melting rock samples, for example—toexplore how granite magma forms and how fast it can move. Their resultsproved surprising. "In the past we thought that granite magma, whichcools and crystallizes to form very large granite intrusions, moved upthrough kilometers of crust as large, solid blobs at rates of about a meterper year," Cruden says. "But we've found that magma actually has quitelow viscosity and is relatively runny. Because it is runny, it is able tochannel its way from the mantle and lower crust through fractures andcracks that are as small as one meter thick."
According to this model, granite intrusions in Greenland or the CanadianShield, depending on their size, would have taken only thousands of yearsto form, which is extraordinarily fast from a geological point of view, Crudennotes.
The first paragraph of this passage serves primarily to______________
(A) introduce two competing explanations of a natural phenomenon.
(B) establish the validity of evidence supporting a new scientific theory.
(C) compare and contrast two perspectives on a recent discovery.
(D) present a detailed timeline of advances in a field of study.
解析：A是正确的，第一段给出了两种不同的关于大陆形成的解释，一种是百年来逐渐形成的，另一种是最近的研究说是相对快速的从1000到100000年形成的。
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