46. In recent years， there has been a dramatic decline in the population of the shrike， a predatory bird that inhabits flat land， such as farms and pastures. Some ornithologists hypothesize that this decline is due to the introduction of new， more effective pesticides to control the insect species on which shrikes prey.
The answer to which of the following questions is NOT relevant to evaluating the ornithologists' hypothesis?
(A) Was there a decline in the shrike population before the new pesticides were first used?
(B) Have shrike populations declined significantly in those habitats where the new pesticides have not been used?
(C) Have the new pesticides more significantly reduced the population of insect species on which shrikes prey than did the pesticides previously used?
(D) Are insects that have consumed the new pesti-cides more toxic to the shrikes that eat those insects than were insects that consumed the less effective pesticides?
(E) Are the new pesticides considered by most people to be less harmful to the environment than the old pesticides were considered to be?
47. Census data for Prenland show that unmarried Prenlandic men in their thirties outnumber unmarried Prenlandic women in that age group by about ten to One. Most of these men do wish to marry. Clearly，however， unless many of them marry women who are not Prenlandic， all but a minority will remain unmarried.
The argument makes which of the following assump-tions?
(A) Emigration from Preland is more common among women than among men.
(B) A greater proportion of Prelandic women in their thirties than of Prenlandic men of the same age would prefer to remain unmarried.
(C) It is unlikely that many of these unmarried Prenlandic men will marry women more than a few years older than themselves.
(D) Prenland has a high rate of divorce.
(E) Most of the unmarried Prenlandic men are unwilling to marry women who are not Prenlandic.
48. Certain extremely harmful bacteria found only in sewage are difficult to detect directly. Testing for E. coli， an easily detected and less harmful type of bacteria， in ocean water would be a reliable way of determining whether or not these more harmful bac-teria are present， since ocean water contains E. Coli only if the water is contaminated with sewage that contains the harmful bacteria.
Which of the following， if true， most seriously weakens the argument?
(A) There are many different strains of the E. coli bacteria， and only some of these strains are harmful.
(B) Some types of bacteria found in sewage are neither disease-causing nor difficult to detect directly.
(C) Some of the types of bacteria found in sewage along with E. coli are not harmful to people unless the bacteria are ingested in large quantities.
(D) E. coli dies out much more quickly than some of the more harmful bacteria found in sewage and then can no longer be easily detected.
(E) Some of the types of bacteria found in sewage along with E. coli reproduce at a slower rate than E. coli.
49. The organizers of tomorrow's outdoor concert announced that it will go on tomorrow on schedule unless bad weather is forecast or too few advance tickets are sold. If the concert is canceled， refunds will be made to ticket holders. Since some ticket holders have already been issued refunds even though more than enough advance tickets were sold， it must be the case that bad weather is forecast.
Which of the following is an error of reasoning con-tained in the argument?
(A) It proceeds as if a condition， which by itself is enough to guarantee a certain result， is the only condition under which that result would occur.
(B) It bases a conclusion that is known to require two conditions on evidence that bears on only one of those conditions.
(C) It explains one event as being caused by another event， even though both events must actually have been caused by some third， unidentified event.
(D) It treats evidence for the absence of one condi-tion under which a circumstance would occur as conclusive evidence that that circumstance will not occur.
(E) Evidence given to support the conclusion actually undermines it.
参考答案： 46.E 47.C 48.D 49.A