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OFFICIAL33 Look at the four squares [■] that indicate where the following sentence could be added to the passage. Where would the sentence best fit?

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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. 

13.Look at the four squares [■] that indicate where the following sentence could be added to the passage. Where would the sentence best fit?

This focused on the chemical composition of ancient rocks..

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正确答案:B
题目解析:
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【题目翻译】请看四个方块,它们指示可以在文章中添加以下句子的位置。这句话最适合哪一句? A:这集中在古代岩石的化学成分上。 【判定题型】:根据题目问法,题目要求将句子插入到文中最恰当的空格处,故判断本题为句子插入题。 【待插入句分析】给出的句子中出现了this的指代提示, 表示this指代前文相关内容, 而该句尾提到了hemical composition of ancient rocks,下一句应该与化学组成有关。 【原文分析】原文说了“陨星理论”,陨星使得K-T灭绝成因发生革命性变化。 【选项分析】 A.第一个方框后还未出现对岩石化学成分的叙述,故错误。 B.第二个方框满足条件。在第二个方框之后才出现了对于iridium的讨论。正确。 C.第三个方框前开始了对iridium的叙述,放在第三个方框的话句子顺序就不通顺了,故错误。 D.第四个方框的错误理由和第三个方框相同,同时第四个方框之后已经开始进行总结了。故错误。

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