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六千五百万年前，恐龙迅速灭绝了，这次的大灭绝也就是人们熟知的K-T事件的一部分。因为它是和地质学上的标记K-T边界有关，K-T边界就是可以在世界很多地方都发现的薄薄的沉积带（K就是白垩纪的缩写，起源于德国的名字Kreidezeit）。

Dinosaurs rapidly became extinct about 65 million years ago as part of a mass extinction known as the K–T event, because it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world (K is the traditional abbreviation for the Cretaceous, derived from the German name Kreidezeit).

关于恐龙为什么会灭绝有很多的解释。

Many explanations have been proposed for why dinosaurs became extinct.

比如说，很多人都把恐龙的灭绝怪罪于开花植物的发展，这些开花植物很难消化，会使得恐龙便秘或者是消化不良——但是开花植物是在早期的白垩纪时期进化的，也就是在恐龙灭绝之前的六千万年左右。

For example, some have blamed dinosaur extinction on the development of flowering plants, which were supposedly more difficult to digest and could have caused constipation or indigestion—except that flowering plants first evolved in the Early Cretaceous, about 60 million years before the dinosaurs died out.

事实上，一些科学家指出，有一口复杂磨牙的鸭嘴恐龙和有角恐龙，它们进化是为了利用这种快速生长的开花植物新资源。

In fact, several scientists have suggested that the duckbill dinosaurs and horned dinosaurs, with their complex battery of grinding teeth, evolved to exploit this new resource of rapidly growing flowering plants.

其他人把恐龙的灭绝归咎于和哺乳动物之间的竞争，据称这些哺乳动物吃了所有的恐龙蛋—但是哺乳动物和恐龙都是在后期三叠纪的同一时期出现的，也就是大约一亿九千万年前，况且也没理由去相信在共存了一亿两千年后，哺乳动物突然爱吃恐龙蛋。

Others have blamed extinction on competition from the mammals, which allegedly ate all the dinosaur eggs—except that mammals and dinosaurs appeared at the same time in the Late Triassic, about 190 million years ago, and there is no reason to believe that mammals suddenly acquired a taste for dinosaur eggs after 120 million years of coexistence.

其他的解释（比如有一种说恐龙全部死于疾病）也都是无力的，因为没有办法去科学地证实它们，所以这些解释仅是一种推测和猜想。

Some explanations (such as the one stating that dinosaurs all died of diseases) fail because there is no way to scientifically test them, and they cannot move beyond the realm of speculation and guesswork.

关于恐龙灭绝的解释忽略了一个很重要的点：在白垩纪末期的灭绝是一次全球性的事件，这次灭绝包括食物链最顶端和最底端的生物体。

This focus on explaining dinosaur extinction misses an important point: the extinction at the end of the Cretaceous was a global event that killed off organisms up and down the food chain.

海洋里面的很多种浮游生物都灭绝了，很多以浮游生物为食的处于食物链底端的海洋生物也都灭绝了。

It wiped out many kinds of plankton in the ocean and many marine organisms that lived on the plankton at the base of the food chain.

这包括很多种类的蚌和蜗牛，尤其是菊石，菊石是一种带壳的像是鱿鱼一样的生物，它们大规模生活在中生代的海里，并且在之前的好几次大灭绝中都幸存下来了。

These included a variety of clams and snails, and especially the ammonites, a group of shelled squidlike creatures that dominated the Mesozoic seas and had survived many previous mass extinctions.

K–T事件意味着海洋爬行动物时代的结束，比如说沧龙和蛇颈龙，这些都是曾经生活在海里最大的生物，在鲸鱼出现之前一直是它们统治了海洋。

The K–T event marked the end of the marine reptiles, such as the mosasaurs and the plesiosaurs, which were the largest creatures that had ever lived in the seas and which ruled the seas long before whales evolved.

在陆地上，除了恐龙的消失以外，在陆地植物之间也有一场危机。

On land, there was also a crisis among the land plants, in addition to the disappearance of dinosaurs.

所以任何一个关于食物链底端生物（比如海洋里的浮游生物，陆地上的植物）被破坏的解释都可以更好的说明像恐龙一样的食物链顶端的生物到底发生了什么。

So any event that can explain the destruction of the base of the food chain (plankton in the ocean, plants on land) can better explain what happened to organisms at the top of the food chain, such as the dinosaurs.

相反，所有仅仅局限于恐龙的解释完全漏失了重点。

By contrast, any explanation that focuses strictly on the dinosaurs completely misses the point.

白垩纪灭绝是一个全球性的现象，恐龙只不过是这整个现象中的一部分。

The Cretaceous extinctions were a global phenomenon, and dinosaurs were just a part of a bigger picture.

根据一个理论，恐龙的生存时代突然止于六千五百万年前，也就是一个巨大的岩石从太空骤然跌落到地球上。

According to one theory, the Age of Dinosaurs ended suddenly 65 million years ago when a giant rock from space plummeted to Earth.

这个火球（要么是彗星要么是流星）据推测直径在十到十五千米，每秒的宇宙速度是20—70千米，或者是每小时45，000—156，000英里。

Estimated to be ten to fifteen kilometers in diameter, this bolide (either a comet or an asteroid) was traveling at cosmic speeds of 20–70 kilometers per second, or 45,000–156,000 miles per hour.

这么大的一个以极其快的速度移动的物体，携带着巨大的能量。

Such a huge mass traveling at such tremendous speeds carries an enormous amount of energy.

当火球撞击的时候，里面的能量就被释放出来，产生出来了巨大的冲击波，这些冲击波夷平了撞击处周围数千公里的土地，使得地面的绝大多数景观都成了火焰。

When the bolide struck, this energy was released and generated a huge shock wave that leveled everything for thousands of kilometers around the impact and caused most of the landscape to burst into flames.

这个火球撞击了被称作是西克苏鲁伯的墨西哥尤卡坦半岛，凿出了一个15—20千米深，至少直径170千米的火山口。

The bolide struck an area of the Yucatán Peninsula of Mexico known as Chicxulub, excavating a crater 15–20 kilometers deep and at least 170 kilometers in diameter.

这份冲击移置了大量的海水，造成了加勒比海地区很多的洪水灾害。

The impact displaced huge volumes of seawater, causing much flood damage in the Caribbean.

同时，这个火球本身也在那个地方凿出了100立方公里的岩石和残骸，把它们卷到了100千米的高度。

Meanwhile, the bolide itself excavated 100 cubic kilometers of rock and debris from the site, which rose to an altitude of 100 kilometers.

它们绝大多数都立刻掉回了地上，但是还有一些作为灰尘留在了大气层中长达数月。

Most of it fell back immediately, but some of it remained as dust in the atmosphere for months.

这些物质，同火产生的烟一起，覆盖住了地球，产生了一种核子冬天的样子。

This material, along with the smoke from the fires, shrouded Earth, creating a form of nuclear winter.

根据电脑合成的气候模型，全球气温低至了将近冰点，光合作用暂停了，陆地和海里的绝大多数植物都死掉了。

According to computerized climate models, global temperatures fell to near the freezing point, photosynthesis halted, and most plants on land and in the sea died.

随着食物链底端的生物收到摧毁，恐龙也不可能活下来了。

With the bottom of the food chain destroyed, dinosaurs could not survive.

In paragraph 1, why does the author include a discussion of when flowering plants evolved?

According to paragraph 1, the extinction of the dinosaurs is unlikely to have been the result of competition from mammals because

According to paragraph 2, what is problematic about some scientists’ focus on dinosaur extinction?

According to paragraph 2, each of the following became extinct during the K–T event EXCEPT

The word “crisis” in the passage is closest in meaning to

How does paragraph 3 relate to paragraph 2?

Paragraph 3 answers all of the following questions EXCEPT:

Paragraph 3 strongly suggests that if the bolide impact theory is correct, the majority of the extinctions associated with the K–T event resulted from

Look at the four squares  that indicate where the following sentence could be added to the passage. Some explanations seem plausible until the facts are considered.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Explaining Dinosaur Extinction Dinosaurs rapidly became extinct about 65 million years ago as part of a mass extinction known as the K–T event, because it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world (K is the traditional abbreviation for the Cretaceous, derived from the German name Kreidezeit).  Many explanations have been proposed for why dinosaurs became extinct.  For example, some have blamed dinosaur extinction on the development of flowering plants, which were supposedly more difficult to digest and could have caused constipation or indigestion—except that flowering plants first evolved in the Early Cretaceous, about 60 million years before the dinosaurs died out.  In fact, several scientists have suggested that the duckbill dinosaurs and horned dinosaurs, with their complex battery of grinding teeth, evolved to exploit this new resource of rapidly growing flowering plants.  Others have blamed extinction on competition from the mammals, which allegedly ate all the dinosaur eggs—except that mammals and dinosaurs appeared at the same time in the Late Triassic, about 190 million years ago, and there is no reason to believe that mammals suddenly acquired a taste for dinosaur eggs after 120 million years of coexistence. Some explanations (such as the one stating that dinosaurs all died of diseases) fail because there is no way to scientifically test them, and they cannot move beyond the realm of speculation and guesswork. This focus on explaining dinosaur extinction misses an important point: the extinction at the end of the Cretaceous was a global event that killed off organisms up and down the food chain. It wiped out many kinds of plankton in the ocean and many marine organisms that lived on the plankton at the base of the food chain. These included a variety of clams and snails, and especially the ammonites, a group of shelled squidlike creatures that dominated the Mesozoic seas and had survived many previous mass extinctions. The K–T event marked the end of the marine reptiles, such as the mosasaurs and the plesiosaurs, which were the largest creatures that had ever lived in the seas and which ruled the seas long before whales evolved. On land, there was also a crisis among the land plants, in addition to the disappearance of dinosaurs. So any event that can explain the destruction of the base of the food chain (plankton in the ocean, plants on land) can better explain what happened to organisms at the top of the food chain, such as the dinosaurs. By contrast, any explanation that focuses strictly on the dinosaurs completely misses the point. The Cretaceous extinctions were a global phenomenon, and dinosaurs were just a part of a bigger picture. According to one theory, the Age of Dinosaurs ended suddenly 65 million years ago when a giant rock from space plummeted to Earth. Estimated to be ten to fifteen kilometers in diameter, this bolide (either a comet or an asteroid) was traveling at cosmic speeds of 20–70 kilometers per second, or 45,000–156,000 miles per hour. Such a huge mass traveling at such tremendous speeds carries an enormous amount of energy. When the bolide struck, this energy was released and generated a huge shock wave that leveled everything for thousands of kilometers around the impact and caused most of the landscape to burst into flames. The bolide struck an area of the Yucatán Peninsula of Mexico known as Chicxulub, excavating a crater 15–20 kilometers deep and at least 170 kilometers in diameter. The impact displaced huge volumes of seawater, causing much flood damage in the Caribbean. Meanwhile, the bolide itself excavated 100 cubic kilometers of rock and debris from the site, which rose to an altitude of 100 kilometers. Most of it fell back immediately, but some of it remained as dust in the atmosphere for months. This material, along with the smoke from the fires, shrouded Earth, creating a form of nuclear winter. According to computerized climate models, global temperatures fell to near the freezing point, photosynthesis halted, and most plants on land and in the sea died. With the bottom of the food chain destroyed, dinosaurs could not survive.