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The question of why dinosaurs became extinct has puzzled paleontologists sin

游客2025-04-03  36
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. [br] The word "elusive" in Paragraph 1 most nearly means______.
选项 A、evasive
B、hard to understand
C、difficult to find
D、rare
答案 C
解析 语义题。A．evasive“逃避的，转弯抹角的”，如an evasive talk(含义模糊的谈话)，B“难以理解的”；C“难找的”；D“稀有的，罕见的”；elusive“难找的”，故选C。
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