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16. The presence of microorganisms that produce a toxin cause sweater to turn brownish red, phenomenon known as a red ride. Sea otters do not feed in areas where clams, their main source of food, have become contaminated with this toxin. According to a proposed explanation of the otter’s behavior, the otters sample the clams in a potential feeding area and can taste any toxin in them.
  
Which of the following, if true, would most strongly indicate that the hypothesis described in the last sentence of the passage is not correct?

(A) In some of the area where red tides occur, neither clams nor sea otters are indigenous species.
(B) The presence of sea otters in a given area has a significant effect on which other marine organisms are to be found in that areas.
(C) When seawater in an area unaffected by red tide is artificially dyed brownish red, sea otters do not feed on the clams in that area.
(D) If the clams in a given areas are contaminated with toxin, sea otters move to other areas in search of food.
(E) Although very small amounts of the toxin produced during a red tide are not harmful, large doses can be fatal to animals the size of sea otters.

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17. Since 1945 there have been numerous international confrontations as tense as those that precipitated the Second World War, and yet no large-scale conflict has resulted. To explain this, some argue that fear of enormous destruction such as the Second War produced has had a dramatic deterrent effect.
  
Which of the following, if true, most seriously weakens the deterrent theory mentioned above?

(A) After the First World War, the fear of great future destruction was as intense as it was after the Second World War.
(B) Psychologists have determined that the fear of retaliation tends to temper aggressiveness among human beings.
(C) The Second World War was far less distractive than most people generally believe.
(D) Fear of repeating the levels of destruction that the Second World War produced is as pervasive today as it was forty years ago.
(E) Many of the international confrontations that have occurred since 1945 have involved countries that participated in the Second World War.

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18. A conservation group in the United States is trying to change the long-standing image of bats as frightening creatures. The group contends that bats are feared and persecuted solely because they are shy animals that are active only at night.

Which of the following, if true, would cast the most serious doubt on the accuracy of the group’s contention?
A. Bats are steadily losing natural roosting places such as caves and hollow trees and are thus turning to more developed areas for roosting.
B. Bats are the chief consumers of nocturnal insects and thus can help make their hunting territory more pleasant for humans.
C. Bats are regarded as frightening creatures not only in the United States but also in Europe, Africa, and South America.
D. Raccoons and owls are shy and active only at night; yet they are not generally feared and persecuted.
E. People know more about the behavior of other greatly feared animal species, such as lions, alligators, and greatly feared animal species, such as lions, alligators, and snakes, than they do about the behavior of bats.

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19.(Z13)Healthy lungs produce a natural antibiotic that protects them from infection by routinely killing harmful bacteria on airway surfaces. People with cystic fibroses, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic. Since the fluid on airway surfaces in the lungs of people with cystic fibrosis bas an abnormally high salt concentration, scientists hypothesize that in high salt environments the antibiotic becomes ineffective at killing harmful bacteria.
Which of the following, if it were obtained as an experimental result, would most decisively undermine the scientists’ hypothesis?
A.        Healthy lungs in which the salt concentration of the airway-surface fluid has been substantially increased are able to reestablish their normal salt concentration within a relatively short period of time.
B.        The antibiotic produced by the lungs is effective at killing harmful bacteria even when salt concentrations are below levels typical of healthy lungs.
C.        The salt concentration of the airway-surface fluid in the lungs of people who suffer from cystic fibrosis tends to return to its former high levels after having been reduced to levels typical of healthy lungs.
D.        The lungs of people who suffer from cystic fibrosis are unable to fight off harmful bacteria even when the salt concentration is reduced to levels typical of healthy lungs.
E.        The salt concentration in the airway-surface fluid of people whose lungs produce lower-than-average amounts of the antibiotic is generally much lower than that typical of healthy lungs.

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20. The number of people diagnosed as having a certain intestinal disease has dropped significantly in a rural county this year, as compared to last year. Health officials attribute this decrease entirely to improved sanitary conditions at water-treatment plants, which made for cleaner water this year and thus reduced the incidence of the disease.
Which of the following, if true, would most seriously weaken the health officials' explanation for the lower incidence of the disease?
(A)        Many new water-treatment plants have been built in the last five years in the rural county.
(B)        Bottled spring water has not been consumed in significantly different quantities by people diagnosed as having the intestinal disease, as compared to people who did not contract the disease.
(C)        Because of a new diagnostic technique, many people who until this year would have been diagnosed as having the intestinal disease are now correctly diagnosed as suffering from intestinal ulcers.
(D)        Because of medical advances this year, far fewer people who contract the intestinal disease will develop severe cases of the disease.
(E)        The water in the rural county was brought up to the sanitary standards of the water in neighboring counties 10 years ago.

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21. The price of maple syrup has jumped from 22 dollars a gallon three years ago to 40 dollars a gallon today. It can be concluded that maple-syrup harvesters have been artificially inflating prices and that governmental price regulations are necessary to control rising prices.

Which of the following, if true, casts the most doubt on the conclusion drawn above?

(A) The government already requires maple-syrup harvesters to submit their facilities to licensing by
the health department.
(B) Insect infestation and drought have stunted the growth of syrup-producing maple trees and caused less-abundant syrup harvests.
(C) Maple syrup is produced in rural areas that suffer from high unemployment.
(D) Technological improvements in maple-syrup harvesting have reduced production costs.
(E) Maple-syrup prices have risen many times in the past, though never before at the rate recently observed.

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22. (Z3)Guidebook writer:  I have visited hotels throughout the country and have noticed that in those built before 1930 the quality of the original carpentry work is generally superior to that in hotels built afterward.  Clearly carpenters working on hotels before 1930 typically worked with more skill, care, and effort than carpenters who have worked on hotels built subsequently.

Which of the following, if true, most seriously weakens the guidebook writer’s argument?

A.        The quality of original carpentry in hotels is generally far superior to the quality of original carpentry in other structures, such as houses and stores.
B.        Hotels built since 1930 can generally accommodate more guests than those built before 1930.
C.        The materials available to carpenters working before 1930 were not significantly different in quality from the materials available to carpenters working after 1930.
D.        The better the quality of original carpentry in a building, the less likely that building is to fall into disuse and be demolished.
E.        The average length of apprenticeship for carpenters has declined significantly since 1930.

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23.(Z5) In the late 1980’s, the population of sea otters in the North Pacific Ocean began to decline. Of the two plausible explanations for the decline—increased predation by killer whales or disease—disease is the more likely. After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably.

Which of the following, if true, most seriously weakens the reasoning?

A.        Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.
B.        There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980’s.
C.        Along the Pacific coast of North America in the 1980’s, sea otters were absent from many locations where they had been relatively common in former times.
D.        Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters’ main food source.
E.        The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.

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练习题：
1.（Z1-13）In the United States, of the people who moved from one state to another when they retired, the percentage who retired to Florida has decreased by three percentage points over the past ten years.  Since many local businesses in Florida cater to retirees, this decline is likely to have a noticeably negative economic effect on these businesses.

Which of the following, if true, most seriously weakens the argument?

A. Florida attracts more people who move from one state to another when they retire than does any other state.
B. The number of people who move out of Florida to accept employment in other states has increased over the past ten years.
C. There are far more local businesses in Florida that cater to tourists than there are local businesses that cater to retirees.
D. The total number of people who retired and moved to another state for their retirement has increased significantly over the past ten years.
E. The number of people who left Florida when they retired to live in another state was greater last year than it was ten years ago.

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2.(Z3)Kate:  The recent decline in numbers of the Tennessee warbler, a North American songbird that migrates each fall to coffee plantations in South America, is due to the elimination of the dense tree cover that formerly was a feature of most South American coffee plantations.

Scott:  The population of the spruce budworm, the warbler’s favorite prey in North America, has been dropping.  This is a more likely explanation of the warbler’s decline.

Which of the following, if true, most seriously calls Scott’s hypothesis into question?

A.        The numbers of the Baltimore oriole, a songbird that dose not eat budworms but is as dependent on South American coffee plantations as is the Tennessee warbler, are declining.
B.        The spruce-budworm population has dropped because of a disease that can infect budworms but not Tennessee warblers.
C.        The drop in the population of the spruce budworm is expected to be only temporary.
D.        Many Tennessee warbler have begun migrating in the fall to places other than traditional coffee plantations.
E.        Although many North American songbirds have declined in numbers, no other species has experienced as great a decline as has the Tennessee warbler.