官方真题Official5托福阅读Passage3原文文本
The Cambrian Explosion
The geologic timescale is marked by significant geologic and biological events, including the origin of Earth about 4.6 billion years ago, the origin of life about 3.5 billion years ago, the origin of eukaryotic life-forms (living things that have cells with true nuclei) about 1.5 billion years ago, and the origin of animals about 0.6 billion years ago. The last event marks the beginning of the Cambrian period. Animals originated relatively late in the history of Earth—in only the last 10 percent of Earth’s history. During a geologically brief 100-million-year period, all modern animal groups (along with other animals that are now extinct) evolved. This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.”
Scientists have asked important questions about this explosion for more than a century. Why did it occur so late in the history of Earth? The origin of multicellular forms of life seems a relatively simple step compared to the origin of life itself. Why does the fossil record not document the series of evolutionary changes during the evolution of animals? Why did animal life evolve so quickly? Paleontologists continue to search the fossil record for answers to these questions.
One interpretation regarding the absence of fossils during this important 100-million-year period is that early animals were soft bodied and simply did not fossilize. Fossilization of soft-bodied animals is less likely than fossilization of hard-bodied animals, but it does occur. Conditions that promote fossilization of soft-bodied animals include very rapid covering by sediments that create an environment that discourages decomposition. In fact, fossil beds containing soft-bodied animals have been known for many years.
官方真题Official5托福阅读Passage3解析
Question 1 of 14
正确答案：B
题目解析：B significant: 显著的，重要的。A是许多的;B是重要的;C是未经解释的;D是突然的。答案B的important正确。
Question 2 of 14
正确答案：C
题目解析：relatively: 相对地。A是令人意外地;B是共同地;C是相对地;D是典型地。所以comparatively相对地正确。对应原文：Animals originated relatively late in the history of Earth—in only the last 10 percent of Earth’s history.原句relatively late之后有个破折号对这个进行了解释，说地球历史的最后10%，当然是相对靠后的。A吃惊B聚集D有特点都不对。
Question 3 of 14
正确答案：A
题目解析：diversification: 分化。想到diverse多样，所以A出现了很多变体正确。对应原文：This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.”原文与之并列的是origin(起源)，带有出现的意思，所以emergence对得上;B的decline和D的disappear都说反了;C的body size与原文无关。
Question 4 of 14
正确答案：B
题目解析：B 以explosion做关键词定位至第一段最后一句，注意这道题的相关信息不在第二段而是在第一段。说rapid origin and diversification所以叫explosion，所以B正确。A的时间不能作为explosion的依据;C灭绝说反了;D火山爆发没说。
Question 5 of 14
正确答案：A
题目解析：A EXCEPT题，排除法。A的simple step做关键词定位至第三句，但这句话不是一个问题，所以A错，选;B选项的so long对应原文第二句的so late，所以正确，不选，注意不要单纯以multicellular organism做关键词，会导致定位错误;C对应倒数第二句，正确，不选;D的lack evidence对应原文倒数第三句的not document，正确，不选
Question 6 of 14
正确答案：B
题目解析：B 问整段的题，注意各段的开头句;第二段提出了很多问题，而第三段一开始就给了一个explanation，说明是第二段问第三段答，所以B正确。第三段与第二段没有对比或者反对的关系，所以A和C都不对;也不是详细阐述，D错。
Question 7 of 14
正确答案：C
题目解析：promote: 促进，提升。所以C的encourage正确。A是使…复杂;B是防止;D是影响。对应原文：Conditions that promote fossilization of soft-bodied animals include very rapid covering by sediments that create an environment that discourages decomposition.原文的condition应该是对fossilization起到正面的作用，B的prevent和D的affect都是负面作用，所以都不对;complicate是使变复杂，原文没有给出相关信息，错。
Question 8 of 14
正确答案：C
题目解析：C A的Precambrian对应第二句，正确，不选;C的相关信息也来自第二句，但第二句说的是worldwide in distribution，不是只在澳大利亚有，所以C说反了，选：
Question 9 of 14
正确答案：B
题目解析：B 原句最核心的关系是转折，后半句是作者的真实意思。转折之后说：大部分的古生物学家同意这些化石代表着非常特别的body forms，在Cambrian早期出现，在末期消亡，没有留下子孙后代。D项没有确切结论(从whether看出)，原文有，所以D错;原文后半句说T出现在C早期，晚期就没了，也没有后代，A和C都说有后代，错;所以答案B正确。
Question 10 of 14
正确答案：D
题目解析：Anomalocaris和Wiwaxia是两种unassignable动物的例子，往前看：These fossil beds provide evidence of about 32 modern animal groups, plus about 20 other animal body forms that are so different from any modern animals that they cannot be assigned to any one of the modern groups.前句很清楚说有些动物是不能被分在任何一个group里，接着就给了这两种作为例子，所以D正确，其他都没说。
Question 11 of 14
正确答案：C
题目解析：C 也是一道功能目的题，前句说BS有很多已经灭绝的动物，它们是现代动物group的代表;然后就=给出了S这个例子，所以C正确;不要受到B的干扰，原文说S是一种以前不知道的anthropod的代表，没说是以前不知道的BS的代表，所以错。
Question 12 of 14
正确答案：B
题目解析：B 以Cambrian Explosion做关键词定位至第二句，注意倒数第二句也有一个Cambrian Explosion，但因为与问题无关，所以不读。本句说寒武纪大爆发包含了很多动物的快速出现然后快速灭亡，接着提出问题为什么这么快并给出解答。所以问题中的unusual就是说快的不正常，所以答案B正确。A的new ecological niches原文没有相关信息;C原文只是一个问题，C变成了答案，不对;D错，原文最后一句说一个bed，不是很多beds，而且是awaiting discovery，不是已经发现了。
Question 13 of 14
正确答案：B
题目解析：B两个过渡点，fossilization of soft-bodied animals和special environment，凭第一个A/B/C都有可能，但special environment说明A不对，而且应该先说需要一个特殊的环境然后再说这个环境是什么，而不是反过来。何况B点之后的condition和插入句中的environment刚好对应。
Question 14 of 14
正确答案：ABF
题目解析：A.B.F little选项对应原文第三段第一句，正确。while选项对应原文第五段最后一句和第一段倒数第二句，正确。the Cambrian选项把原文的两个不相干的概念杂糅到一起，胡编乱造，不选。the Edicara选项对应第四段第一句，但因为第四段整个是个例子，所以这个是细节，不选。zoologist选项对应第七段最后一句，但是是细节，不选。although选项对应原文第七段倒数第三句，正确。
