限时练(二) 首段过渡类
30minutes_ Starting time：______________
本节提供4篇首段过渡类文章，限时30分钟。这类文章多以记叙文或者夹叙夹议的文体出现，文章首段通过提问题、记叙事件的方式引导读者进入后面的主题，所以首段起到导引或者过渡作用，本文主题是在其他段落，比如第二段或者最后段落呈现，或者不明说，而需要读者归纳总结。
A 2019·天津卷C篇
How does an ecosystem(生态系统)work？What makes the populations of different species the way they are？Why are there so many flies and so few wolves？To find an answer，scientists have built mathematical models of food webs，noting who eats whom and how much each one eats.
With such models，scientists have found out some key principles operating in food webs. Most food webs，for instance，consist of many weak links rather than a few strong ones. When a predator(掠食动物)always eats huge numbers of a single prey(猎物)，the two species are strongly linked；when a predator lives on various species，they are weakly linked. Food webs may be dominated by many weak links because that arrangement is more stable over the long term. If a predator can eat several species，it can survive the extinction(灭绝)of one of them. And if a predator can move on to another species that is easier to find when a prey species becomes rare，the switch allows the original prey to recover. The weak links may thus keep species from driving one another to extinction.
Mathematical models have also revealed that food webs may be unstable，where small changes of top predators can lead to big effects throughout entire ecosystems. In the 1960s，scientists proposed that predators at the top of a food web had a surprising amount of control over the size of populations of other species—including species they did not directly attack.
And unplanned human activities have proved the idea of top-down control by top predators to be true. In the ocean，we fished for top predators such as cod on an industrial scale，while on land，we killed off large predators such as wolves. These actions have greatly affected the ecological balance.
Scientists have built an early-warning system based on mathematical models. Ideally，the system would tell us when to adapt human activities that are pushing an ecosystem toward a breakdown or would even allow us to pull an ecosystem back from the borderline. Prevention is key，scientists say because once ecosystems pass their tipping point(临界点)，it is remarkably difficult for them to return.
[分析] 这是一篇说明文。本文介绍了科学家们借助食物网的数学模型，发现了食物网中的一些重要现象。科学家们认为，一旦一个生态系统越过了它的临界点，就很难再恢复。
该模型将告诉我们何时调整人类活动，以避免将生态系统推向崩溃，或者我们要将生态系统从边缘拉回来，预防是关键。
1．What have scientists discovered with the help of mathematical models of food webs? A．The living habits of species in food webs. B．The rules governing food webs of the ecosystems. C．The approaches to studying the species in the ecosystems. D．The differences between weak and strong links in food webs.
B [细节理解题。根据题干关键信息discovered with the help of mathematical models of food webs定位到第一段。再由With such models, scientists have found out some key principles operating in food webs. 可知，借助食物网的数学模型，科学家们发现了食物网中的一些关键原则。]
[错项分析] 忽略定位方法，而试图从全文寻找答案，是本题出错和浪费时间的原因。题干的discovered对应文中的found out，mathematical models of food webs则在文中出现，所以定位后确定的答案才准确无误。
2．A strong link is found between two species when a predator______________. A．has a wide food choice C．sticks to one prey species
B．can easily find new prey
D．can quickly move to another place
C [细节理解题。根据题干关键信息predator，two species, strong link可以定位到第二段。根据段中When a predator(掠食动物) always eats huge numbers of a single prey(猎物)，the two species are strongly linked可知，当捕食者总是吃大量的单一猎物，这两个物种是紧密相连的。]
[错项分析] 本题考查选项C．sticks to one prey species和文中always eats huge numbers of a single prey的近义表达，理解不到这一点，就会错选其他。
3．What will happen if the populations of top predators in a food web greatly decline? A．The prey species they directly attack will die out.
B．The species they indirectly attack will turn into top predators. C．The living environment of other species will remain unchanged. D．The populations of other species will experience unexpected changes.
D [推理判断题。根据题干关键信息top predators，populations ，decline定位到第三段，由句子In the 1960s, scientists proposed that predators at the top of a food web had a surprising amount of control over the size of populations of other species可知，处于食物网顶端的食肉动物对其他物种的种群数量有着惊人的控制。由此可推断出，如果食物链顶级食肉动物的数量大大下降，其他物种的种群将经历意想不到的变化。]
[错项分析] 本题难点在于，推断题是根据一些事实信息进行推断，文中不会直接陈述，这类题要么难以定位，要么定位后也难找到答案。这时需要对信息进行对比归纳，才可以锁
定答案。本题容易错选A、B两项，因为有directly attack和indirectly attack和原文中比较接近， 选A是因为没有理解题干decline的意义；选B是没有理解清楚所定位句子的意义。
4．What conclusion can be drawn from the examples in Paragraph 4? A．Uncontrolled human activities greatly upset ecosystems. B．Rapid economic development threatens animal habitats. C．Species of commercial value dominate other species. D．Industrial activities help keep food webs stable.
A [推理判断题。根据题干容易定位到第四段。段中例子一定是支持段落主题句的。根据例子In the ocean，we fished for top predators such as cod on an industrial scale，while on land，we killed off large predators such as wolves. 可知，人类对顶层鱼类和动物的大肆捕杀，极大的影响了生态系统。]
[错项分析] 其他三项虽然也有道理，但是不是本段要描述的内容。抛开所处语境文本盲目猜测是本题出错的原因，如B项 economic development、C项Species of commercial value、D项的 Industrial activities 都未曾在本段提及。
5．How does an early-warning system help us maintain the ecological balance? A．By getting illegal practices under control. B．By stopping us from killing large predators.
C．By bringing the broken-down ecosystems back to normal. D．By signaling the urgent need for taking preventive action.
D [细节理解题。根据early-warning system定位最后一段。段中句子Scientists have built an early-warning system based on mathematical models. Ideally the system would tell us when to adapt human activities that are pushing an ecosystem toward a breakdown or would even allow us to pull an ecosystem back from the borderline. Prevention is key...可知，早期预警系统会告诉我们何时调整人类活动，采取预防措施以避免生态系统崩溃。]
[错项分析] 易错选C项 By bringing the broken-down ecosystems back to normal. 。题目问的是“预警系统是如何帮助我们保持生态平衡的”，根据上面解析，其作用就是警醒，人们可以提前预防。这个系统本身并不能“恢复崩溃的生态系统”。
B 2019·天津卷D篇
Would you BET on the future of this man？He is 53 years old. Most of his adult life has been a losing struggle against debt and misfortune. A war injury has made his left hand stop functioning，and he has often been in prison. Driven by heaven-knows-what motives，he determines to write a book.
The book turns out to be one that has appealed to the world for more than 350 years. That former prisoner was Cervantes，and the book was Don Quixote《堂吉诃德》(). And the story poses
2020届高三高考二轮复习英语限时练2首段过渡类(含答案)
