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OFFICIAL33 According to paragraph 4, which of the following statements explains the importance of the discovery of high levels of iridium rocks?

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Extinction Episodes of The Past
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It was not until the Cambrian period, beginning about 600 million years ago, that a great proliferation of macroscopic species occurred on Earth and produced a fossil record that allows us to track the rise and fall of biodiversity. Since the Cambrian period, biodiversity has generally risen, but there have been some notable exceptions. Biodiversity collapsed dramatically during at least five periods because of mass extinctions around the globe. The five major mass extinctions receive most of the attention, but they are only one end of a spectrum of extinction events. Collectively, more species went extinct during smaller events that were less dramatic but more frequent. The best known of the five major extinction events, the one that saw the demise of the dinosaurs, is the Cretaceous-Tertiary extinction.

Starting about 280 million years ago, reptiles were the dominant large animals in terrestrial environments. In popular language this was the era “when dinosaurs ruled Earth,” when a wide variety of reptile species occupying many ecological niches. However, no group or species can maintain its dominance indefinitely, and when, after over 200 million years, the age of dinosaurs came to a dramatic end about 65 million years ago, mammals began to flourish, evolving from relatively few types of small terrestrial animals into the myriad of diverse species, including bats and whales, that we know today. Paleontologists label this point in Earth’s history as the end of the Cretaceous period and the beginning of the Tertiary period, often abbreviated as the K-T boundary. This time was also marked by changes in many other types of organisms. Overall, about 38 percent of the families of marine animals were lost, with percentages much higher in some groups Ammonoid mollusks went from being very diverse and abundant to being extinct. An extremely abundant set of planktonic marine animals called foraminifera largely disappeared, although they rebounded later. Among plants, the K-T boundary saw a sharp but brief rise in the abundance of primitive vascular plants such as ferns, club mosses, horsetails, and conifers and other gymnosperms. The number of flowering plants (angiosperms) was reduced at this time, but they then began to increase dramatically.

What caused these changes? For many years scientists assumed that a cooling of the climate was responsible, with dinosaurs being particularly vulnerable because, like modern reptiles, they were ectothermic (dependent on environmental heat, or cold-blooded). It is now widely believed that at least some species of dinosaurs had a metabolic rate high enough for them to be endotherms (animals that maintain a relatively consistent body temperature by generating heat internally). Nevertheless, climatic explanations for the K-T extinction are not really challenged by the ideas that dinosaurs may have been endothermic, because even endotherms can be affected by a significant change in the climate.

Explanations for the K-T extinction were revolutionized in 1980 when a group of physical scientists led by Luis Alvarez proposed that 65 million years ago Earth was stuck by a 10-kilometer-wide meteorite traveling at 90,000 kilometers per hour. They believed that this impact generated a thick cloud of dust that enveloped Earth, shutting out much of the incoming solar radiation and reducing plant photosynthesis to very low levels. Short-term effects might have included huge tidal waves and extensive fires. In other words, a series of events arising from a single cataclysmic event caused the massive extinctions. Initially, the meteorite theory was based on a single line of evidence. At locations around the globe, geologists had found an unusually high concentration of iridium in the layer of sedimentary rocks that was formed about 65 million years ago. Iridium is an element that is usually uncommon near Earth’s surface, but it is abundant in some meteorites. Therefore, Alvarez and his colleagues concluded that it was likely that the iridium in sedimentary rocks deposited at the K-T boundary had originated in a giant meteorite or asteroid. Most scientist came to accept the meteorite theory after evidence came to light that a circular formation, 180 kilometers in diameter and centered on the north coast of the Yucatan Peninsula, was created by a meteorite impact about 65 million years ago. 

10.According to paragraph 4, which of the following statements explains the importance of the discovery of high levels of iridium rocks?

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【题目翻译】根据第4段,以下哪项陈述解释了发现高含量铱岩石的重要性? A:它提供了过度暴露于太阳辐射导致K-T灭绝的证据。 B:这表明导致K-T灭绝的原因不止一次。 C:这表明K-T灭绝的原因可能是陨石撞击地球。 D:它提供了K-T灭绝发生在6500万年前的证据。 【判定题型】:题目问的是文章中的具体细节信息,故根据题目问法可以判断本题为事实信息题。 【关键词定位】用high levels of iridium rocks定位到:At locations around the globe, geologists have found an unusually high concentration of iridium in the layer of sedimentary rocks that was formed about 65 million years ago. 意思是“在世界各地的很多地方,地质学家发现铱元素在六千五百万年前的沉积岩层中含量异常丰富。” 【逻辑分析】定位句这句话中并没有说出它的importance, 可以在这句话附近寻找答案,下文中这句话就是答案出处: Therefore, Alvarez and his colleagues concluded that it was likely that the iridium in sedimentary rocks deposited at the K-T boundary had originated in a giant meteorite or asteroid. Most scientist came to accept the meteorite theory after evidence came to light that a circular formation, 180 kilometers in diameter and centered on the north coast of the Yucatan Peninsula, was created by a meteorite impact about 65 million years ago. 这里说了这么高浓度的iridium (铱)可能是来自于大的流星或小行星。因此其重要性就是说明该撞击理论是物种大灭绝的原因。 【选项分析】 A.高浓度的iridium (铱)可能是来自于大的流星或小行星,说明了物种大灭绝可能是因为陨石撞击地球,故选项错误。 B.文中只提及了物种大灭绝可能是因为陨石撞击地球,也未说原因到底有几次。故选项错误。 C.符合文意,故正确。 D.六千五百万年前陨星撞击发生,但是文章未提及是否K-T灭绝发生在6500万年前。故选项错误。

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