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18. 恐龙灭绝和行星撞地球 Iridium, a hard, whitish metal similar to platinum, is extremely rare on Earth. Extremely high concentrations of iridium on Earth result from only two scenarios: massive volcanic eruptions that release iridium from deep within the Earth and meteorites that shower down on Earth from space. When scientists found concentrations of iridium 30 times higher than normal in rock stratum from 65 million years ago, they concluded that a massive meteor or comet hit the Earth and caused the massive extinction of the dinosaurs. Which of the following, if true, most strongly supports the scientists conclusion? A. Volcanoes massive enough to generate high concentrations of iridium are very rare. B. Massive volcanic eruptions occurred frequently 80 million years ago. C. Most scientists support the hypothesis that a cosmic impact wiped out the dinosaurs. D. The massive extinction that occurred 70 million years ago killed not only the dinosaurs but also 70 percent of all life on Earth. E. A comet struck the earth some 120 million years ago, but no widespread extinction occurred. | 
