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[机经题库答案] 托福阅读生物类真题Extinctions at the End of the Cretaceous原文+题目汇总

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发表于 2017-5-9 17:46 |显示全部楼层
类别:生物类
真题150614CN-P3
Title:Extinctions at the End of the Cretaceous
It has long been recognized that  the dinosaurs disappeared from the fossil record at the end of the Cretaceous period (65 million  years ago), and as more knowledge has been gained, we have learned that many other organisms disappeared at  about the same time. The microscopic  plankton (free-floating plants and animals) with calcareous shells suffered  massively. The foundation of the  major marine food chain that led from the minute plankton to shelled animals to large marine  reptiles had collapsed.

  
On land it was not only the large animals that became extinct.  The mammals, most of which were  small, lost some 35 percent of their species worldwide. Plants were also  affected. For example, in North  America 79 percent did not survive, and it has been noted that the survivors were often deciduousthey  could lose their leaves and shut downwhile others could survive as seeds. As in the sea, it seems that on the  land one key food chain collapsed: the  one with leaves as its basic raw material. These leaves were the food of some  of the mammals and of the  herbivorous dinosaurs, which in turn were fed on by the carnivorous dinosaurs. Furthermore, it  is most likely that these large dinosaurs had slow rates of reproduction, which always increases  the risk of extinction. Crocodiles, tortoises, birds, and insects seem to have been little affected. The two first  named are known to be able to survive  for long periods without food, and both can be scavengers (feed on dead  material). Indeed, with the deaths  of so many other animals and with much dead plant material, the food chain based on detritus would have been  well-supplied. Many insects feed on dead material;  furthermore, most have at least one resting stage in which they are very  resistant to damage. In unfavorable  conditions some may take a long time to develop: there is a record of a beetle larva living in dead wood for  over 40 years before becoming an adult. Some birds were scavengers, but the survival of many lineages is a puzzle.

  
What happened in the  biological story just after these extinctionswhat is found in and just above the boundary layer between  the deposits of the Cretaceous and those of the Tertiary (65¨C2.6 million years ago), termed the K/T boundary    For  a very short period the dominant  microorganisms in marine deposits were usually diatoms and dinoflagellates (both single-celled types of plankton).                         The important feature for the survival of  both these groups was the ability  to form protective cysts (sacs around organisms) that rested on the sea floor.  Above these, in the later deposits, are the remains of other minute plankton,  but the types are quite different  from those of the Late Cretaceous.     In terrestrial deposits a sudden and dramatic increase in fern plant  spores marks the boundary in many parts of the world; ferns are early colonizers of barren landscapes. The fern  spike (sudden increase), as it is termed,  has been found also in some marine deposits (such was the abundance of fern spores blown around the world), and it  occurs in exactly the same layer of deposit where the plankton disappear. We can conclude that the major marine  and terrestrial events occurred simultaneously.

  
Many theories have been put forward for the extinction of the  dinosaurs, but most of them can be dismissed. Since 1980 there have been more  focused, but still controversy-ridden,  investigations. In that year Louis and Walter  Alvarez and colleagues from the University of California published their research on the amounts of  various metals in the boundary between  Cretaceous and Tertiary rocks (K/T boundary) in Italy, Denmark, and New Zealand. They had found, accidentally, that a rare metal, iridium, suddenly  became very abundant exactly at the boundary  and then slowly fell away. This phenomenon, known as the iridium spike, has  now been identified in K/T boundary deposits in over a hundred other sites in  the world. Iridium occurs in meteorites and volcanic material, but in the  latter case it is accompanied by elevated levels of nickel and chromium.  These other metals are not especially abundant at the K/T boundary. The  Alvarezes concluded that the iridium spike was due to a large asteroid that struck  Earth 65 million years ago.
  
1. The  word recognized in the passage is closest  in meaning to
  
A.       suspected
B.         argued
C.        assumed
D.       accepted

  
2. According  to paragraph 1, which of the following was true of small planktonic organisms  at the end of the Cretaceous
  
A.        They lost their main sources of food.
B.         They developed calcareous shells.
C.         They decreased greatly in number as did many  other types of organisms.
D.       They  replaced other minute organisms as a food source in the major marine food chain.
  
3. Which  of the following statements is NOT supported by the information provided in paragraph 2 about extinctions at the  end of the Cretaceous
  
A.        About 35 percent of mammal species were lost.
B.         79 percent of North American plants disappeared.
C.         Most birds, tortoises, and crocodiles escaped extinction.
D.       Deciduous  trees were especially likely to go extinct.
4. According  to paragraph 2, which of the following factors probably contributed to the extinction of the dinosaurs
  
A.        The length of time it took dinosaurs to reproduce
B.         Large quantities of dead material disturbing  their habitats
C.         Increased competition for food from scavengers
D.       An  increase in carnivore populations
  
5. In  paragraph 2, why does the author provide the information that there is a  record of a beetle larva living in  dead wood for over 40 years before becoming an adult
  
A.        To help  explain why insects were less likely to go extinct than other species
B.         To show  that not all species that relied on trees disappeared during the late Cretaceous
C.         To suggest  that insects that lived long ago had much longer life spans than those living today
D.        To support  the claim that conditions at the end of the Cretaceous were highly unfavorable  

  
6. The  word simultaneously in the passage is  closest in meaning to
  
A.       rapidly
B.         repeatedly  
C.         at the same  time  
D.       for different reasons  

  
7. According  to paragraph 3, which of the following is true of plankton after the  extinctions at the end of the  Cretaceous
  
A.        Diatoms and dinoflagellates suddenly became  very rare.  
B.         Single-celled types of plankton were replaced  by more complicated microorganisms.  
C.         The plankton found in later deposits are  closely related to single-celled Late Cretaceous  microorganisms.
D.       The  only types of Late Cretaceous plankton to survive extinction were those  protected by cysts.  

  
8. Which of the sentences below best expresses the  essential information in the highlighted  sentence in the passage. Incorrect  choices change the meaning in important ways or leave out essential information.
  
A.        The fern spike is the term given to this  sudden increase in the abundance of fern spores  blown into the oceans around the world.  
B.         The sudden increase in fern spores occurred  in exactly the same parts of the world where  the plankton was disappearing.
C.         Fern spores have even been found in some  marine deposits from exactly the same layer  as that showing the disappearance of the plankton.  
D.       Most  marine deposits from around the world contain either plankton or a lot of  fern spores but not both together  in the same layer.  

  
9. The  word dismissed in the passage is closest  in meaning to
  
A.       further  explored  
B.         rejected  
C.         completely  revised  
D.       replaced
    
10. The  word elevated in the passage is closest  in meaning to
  
A.       high
B.         varying  
C.         unusual  
D.       adequate  

  
11. According  to paragraph 4, what evidence is there that an asteroid hit Earth
  
A.       The  fact that the metals nickel and chromium were found at sites around the world  
B.         The presence in K/T boundary deposits of  large amounts of a metal that is found in  meteorites  
C.         The fact that iridium amounts decreased at  the same time that the Cretaceous ended and  the Tertiary began  
D.        The differences in the types of metals found  in deposits in Italy, Denmark, and  New Zealand
  
12. Paragraph  4 suggests which of the following about the Alvarezes' theory
  
A.       Their  theory was accepted until 1980, when more focused investigations showed it to be incorrect.
B.         It rules out the possibility that the iridium  spike was the result of volcanic activity.  
C.         Before it can be accepted, more evidence must  be gathered from locations outside Europe  and New Zealand.
D.       Experts  believe the research done by the Alvarezes was too broad.
  
13.   Look at the four squares that indicate where the  following sentence could be added to the  passage.
  
This change in plankton found in marine  deposits is what marks the boundary between  the Cretaceous and the Tertiary.
Where  would the sentence best fit?
Click  on a square to add the sentence to  the passage.
(题目不完整)
  
14. Drag  your choices to the spaces where they belong. To review the passage, click on View Text.
Answer  Choices
  
A.        Aside from the dinosaurs, most of the  organisms affected were very small, such  as single-celled plankton and insects.
B.         Herbivores and carnivores were both affected,  but the hardest-hit group was the scavengers,  including many birds and some mammals.  
C.         Two major  food chains were eliminated, one in the oceans based on plankton, and one on land based on leaves.  
D.       Examinations  of marine and terrestrial deposits show clearly that ocean extinctions occurred well before those on land.  
E.         In geological samples from around the world,  the K/T boundary is marked by a sudden  change in plankton and by a spike in fern  spores.  
F.         Sudden, high levels of iridium found at the  K/T boundary suggest that a meteorite might  have caused the extinctions.

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