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  3. The question of why dinosaurs became extinct has puzzled paleontologists since the first dinosaur fossil was found almost two ce

The question of why dinosaurs became extinct has puzzled paleontologists since the first dinosaur fossil was found almost two ce

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问题     The question of why dinosaurs became extinct has puzzled paleontologists since the first dinosaur fossil was found almost two centuries ago. These great reptiles dominated the earth for almost 160 million years, but mysteriously died out approximately 65 million years ago. Various explanations for this disappearance have been offered, ranging from an epidemic to a sudden, catastrophic drop in temperature, but definitive proof has remained elusive.
    In 1980, Luis Alvarez, winner of the Nobel Prize in Physics, suggested a novel explanation; cosmic extinction. According to Alvarez and his geologist son Walter, a huge meteor crashed into the earth’s surface 65 million years ago, sending up a massive cloud of dust and rock particles. The cloud blocked out sunlight for a period of months or even years, disrupting plant photosynthesis and, by extension, the global food chain. The lack of vegetation, coupled with a significant drop in temperature, resulted in the extinction of the dinosaurs.
    Alvarez based his theory on a curious piece of evidence: the presence of a thin layer of iridium that had recently been discovered in geologic sediments laid down at approximately the time the dinosaurs died out. The metal iridium is rarely found on the earth’s surface; Alvarez reasoned that it had either come up from the earth’s core by volcanic action, or been deposited from space, through the fall of one or more meteorites. He found the latter explanation more likely, given the even distribution of the iridium layer worldwide. But paleontologists scoffed at the Alvarez extinction theory. Neither Luis nor Walter Alvarez was a paleontologist, yet they claimed to have solved a mystery that had defied the efforts of paleontologists for over a century. Professional hostility was also fueled by the somewhat abrasive style of the elder Alvarez. But the most important objection to the Alvarez theory was evidential. In order to create worldwide fallout on the scale suggested by Alvarez, the "doomsday" meteorite would have had to be on the order of five miles in diameter; its impact would have formed a crater perhaps a hundred miles wide. Where was the crater?
    Finally, a decade after the cosmic extinction theory was first proposed, the crater was found. Lying on the northern edge of Mexico’s Yucatan Peninsula, the crater is 110 miles wide. Long buried under sediment, it had actually been discovered in 1981 by oil geologists, but datings of nearby rock samples taken at that time suggested that it was significantly older than 65 million years. New samples of melted rock from the crater itself were recently analyzed by an advanced dating process, however, and were found to be 64. 98 million years old. Many scientists now feel that, thanks to the Alvarez theory, the mystery of dinosaur extinction has finally been solved.
According to the author, the discovery of a layer of iridium in geologic sediments was considered unusual because______.
选项 A、iridium had never been detected there before
B、the metal is normally quite scarce at the earth’s surface
C、few volcanoes had been active during the era when the sediments were laid down
D、nobody had previously thought to link iridium with dinosaur extinction
答案B
解析 细节推断题。A“铱从未被发现过”，第三段第二句话指出，化学元素铱来自于地壳或外太空，说明铱这种元素是被熟悉的，否则作者也不会知道它的分布，故A错误；正是由于铱在地球表面十分罕见，所以在地质沉积物中发现它才显得不同寻常，因此B正确；文章并没有涉及C的内容，故排除；通过文章可以知道，铱和恐龙的灭绝并无关系，D不正确，故选B。
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