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Until recently most scientists thought they knew what killed off the dinosaurs. A 10km-wide meteorite had smashed into the Yucat

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问题     Until recently most scientists thought they knew what killed off the dinosaurs. A 10km-wide meteorite had smashed into the Yucatan peninsula in Mexico, causing worldwide forest fires, tsunamis several kilometers high, and an "impact winter"—in which dust blocked out the sun for months or years. It was thought that the dinosaurs were blasted, roasted and frozen to death, in that order.
    But now a small but vociferous(marked by noisy and vehement outcry)group of scientists believes there is increasing evidence that this "impact" theory could be wrong. That suggestion has generated one of the bitterest scientific rows of recent times. 【C1】______
    For supporters of the impact theory, the KT boundary layers contained two crucial clues. In 1979 scientists discovered that there were high concentrations of a rare element called iridium(铱), which they thought could only have come from an asteroid.
【C2】______
    On the basis of the spherules and a range of other evidence, Dr. Alan Hildebrand concluded that the impact must have happened in the Yucatan peninsula, at the site of a crater(A bowl-shaped depression at the mouth of a volcano)known as Chicxulub. Chemical analysis later confirmed that the spherules had indeed come from rocks within the crater.
【C3】______
    But a group of scientists led by Prof. Gerta Keller and Prof. Wolfgang Stinnesbeck begged to differ. They uncovered a series of geological clues which suggest the truth may be far more complicated. In short, that the crater in the Yucatan is too old to have killed off the dinosaurs.
    They concentrated on a series of rock formations in Mexico where the iridium layer was separated from the spherule layer by many metres of sandstone.
【C4】______
    But Keller’s team found evidence—such as ancient worm burrows—that suggested that the deposition of the sandstone had been interrupted many times. They deduced that there was a gap of some 300, 000 years between the deposition of the spherules(from the Chicxulub crater)and the iridium(from an asteroid). Therefore there must have been two impacts.
【C5】______
    Keller’s views provoked a lively scientific row. Although still in the minority, Keller’s work does now attract some support. The cause of the dinosaur’s decease is still up in the air.
[A]The impact theory was beautifully simple and appealing. Much of its evidence was drawn from a thin layer of rock known as the "KT boundary". This layer is 65 million years old(which is around the time when the dinosaurs disappeared)and is found around the world exposed in cliffs and mines.
[B]That opinion sparked a massive row, as the supporters of the impact theory such as Prof. Jan Smit of Vrije Universiteit, Amsterdam, rubbished Keller’s ideas. Smit argued that the sandstone had been deposited by massive tsunami waves caused by the asteroid, and so did not undermine the idea of a single impact.
[C]The impact theory seemed to provide the complete answer. In many locations around the world, the iridium layer(evidence of an asteroid impact)sits right on top of the spherule layer(evidence that the impact was at Chicxulub). So Hildebrand and other supporters of the impact theory argued that there was one massive impact 65 million years ago, and that it was at Chicxulub. This, they concluded, must have finished off the dinosaurs by a variety of mechanisms.
[D]Moreover,right underneath the iridium was a layer of "spherules", tiny balls of rock, which seemed to have been condensed from rock which had been vaporized by a massive impact.
[E]The Chicxulub impact, they said, was too old to have finished off the dinosaurs, and there must have been another impact somewhere else which was to blame. That crater has not yet been found.
[F]Dinosaurs are reptiles that dominated the terrestrial ecosystem for most of their 165-million year existence. They became extinct 65 million years ago at the end of the Cretaceous period, and are known from fossilized bones, feces, trackways, gastroliths, and in a few cases impressions of skin and internal organs.
[G]Before the late 1970s, there were any number of different theories to explain the mass extinction, with no real consensus. One theory put it that mammals had eaten all the dinosaurs’ eggs.
【C2】
选项
答案D
解析 此处应该承接上文，介绍岩石层提供的另外一条线索：在“铱”元素密集层的下面分布着一层球状的岩粒。这种颗粒似乎是岩石在受到强力冲撞后蒸发浓缩而成的。
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