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  3. In the following article, some sentences have been removed. For Questions 1 — 5, choose the most suitable one from the list A—G

In the following article, some sentences have been removed. For Questions 1 — 5, choose the most suitable one from the list A—G

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问题 In the following article, some sentences have been removed. For Questions 1 — 5, choose the most suitable one from the list A—G to fit into each of the numbered blank. There are two extra choices which you do not need to use.
    Between 5,000 million and 4,000 million years ago the Earth was formed. By 3,000 million years ago life had arisen and we have fossils of microscopic bacteria-like creatures to prove it. 【R1】______. Nobody knows what happened, but theorists agree that the key was the spontaneous arising of self-duplicating entities, i. e. something equivalent to "genes" in the general sense.
    The atmosphere of the early Earth probably contained gases still abundant today on other planets in the solar system. Chemists have experimentally reconstructed these ancient conditions in the laboratory. If plausible gases are mixed in a container with water, and energy is added by an electric discharge(simulated lightning), organic substances are spontaneously synthesized. These include the building blocks of RNA and DNA. It seems probable that something like this happened on the early Earth. Consequently, the sea would have become a "soup" of prebiological organic compounds. 【R2】______.
    Today the most famous self-duplicating molecule is DNA, but it is widely thought that DNA itself could not have been present at the origin of life because its duplication is too dependent on support from specialized machinery, which could not have been available before evolution itself began. Perhaps the related molecule RNA, which still plays various vital roles in living cells, was the original self-duplicating molecule. Or perhaps the earliest duplicator was a different kind of molecule altogether. 【R3】______. Variants that were particularly good at duplication would automatically have come to predominate in the ancient organic soup. Varieties that did not duplicate, or that did so inaccurately, would have become relatively less numerous. This led to ever-increasing efficiency among duplicating molecules.
    As the competition between duplicating molecules warmed up, success must have gone to the ones that happened to hit upon special tricks or devices for their own self-preservation and their own rapid replication. The rest of evolution may be regarded as a continuation of the natural selection of duplicator molecules, now called genes, by virtue of their capacity to build for themselves efficient devices(cells and multicellular bodies)for their own preservation and reproduction. 【R4】______.
    Fossils were not laid down on more than a small scale until nearly 600 million years ago. The first vertebrates may date back 530 million years, according to fossil evidence. Vertebrates appear abundantly in fossil beds between 300 and 400 million years ago. 【R5】______. Mammals and, later, birds, arose from two different branches of reptiles. The rapid divergence of mammals into the rich variety of types that we see today, from anteaters to monkeys, seems to have been released into the vacuum left by the catastrophic extinction of the dinosaurs, 65 million years ago.
[A]Among vertebrates, the land was first colonized by lobe-finned and lung-bearing fish about 250 million years ago, then by amphibians and, in more thorough fashion, by various kinds of animals that we loosely lump together as "reptiles".
[B]Once self-duplicating molecules had been formed by chance, something like Darwinian natural selection could have begun: variation would have come into the population because of random errors in copying.
[C]It is not enough, of course, that organic molecules appeared in the ancient organic soup. The crucial step, as noted above, was the origin of self-duplicating molecules, molecules capable of copying themselves.
[D]Although we naturally emphasize the evolution of our own kind—the vertebrates, the mammals, and the primates—these constitute only a small branch of the great tree of life.
[E]When the environment changes, or when organisms move to a different environment, different variations are selected, leading eventually to different species. Harmful mutations usually die out quickly, so they don’t interfere with the process of beneficial mutations accumulating.
[F]Three thousand million years is a long time, and it seems to have been long enough to have produced such astonishingly complex devices as the vertebrate body and the insect body.
[G]Some time between these two dates—independent molecular evidence suggests about 4,000 million years ago—that mysterious event, the origin of life, must have occurred.
【R5】
选项
答案A
解析 本题空格出现在第五段中。末段主要以化石证据为线索，简单勾勒出生物界由脊椎动物至目前多样化的动物类型的进化过程。因此有理由推测，空格部分应该承上启下，将上文中的“脊椎动物”与下文中的“爬行动物”连接起来，显然正确选项是[A]。该选项梳理了脊椎动物的进化过程，首先是鱼类，然后是两栖动物然后至爬行动物。among vertebrates承接上文，锁定话题范围，The land was first colonized by…then by…and by…依次介绍了各种脊椎动物的进化顺序。另外，[A]中的时间信息250 million years ago也正好将空格上文的600 million years，530 million years，300and 400 million years与下文的65million years串联起来。
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