A recent study has provided clues to predator-prey dynamics in the late Pleistocene era. Researchers compared the number of tooth fractures in present-day carnivores with tooth fractures in carnivores that lived 36,000 to 10,000 years ago and that were preserved in the Rancho La Brea tar pits in Los Angeles. The breakage frequencies in the extinct species were strikingly higher than those in the present-day species.  

　　In considering possible explanations for this finding, the researchers dismissed demographic bias because older individuals were not overrepresented in the fossil samples. They rejected preservational bias because a total absence of breakage in two extinct species demonstrated that the fractures were not the result of abrasion within the pits. They ruled out local bias because breakage data obtained from other Pleistocene sites were similar to the La Brea data. The explanation they consider most plausible is behavioral differences between extinct and present-day carnivores—in particular, more contact between the teeth of predators and the bones of prey due to more thorough consumption of carcasses by the extinct species. Such thorough carcass consumption implies to the researchers either that prey availability was low, at least seasonally, or that there was intense competition over kills and a high rate of carcass theft due to relatively high predator densities. Questions 4–8 refer to the passage above. 

　　4. The primary purpose of the passage is to (主旨) (A) present several explanations for a well-known fact  

　　(B) suggest alternative methods for resolving a debate  

　　(C) argue in favor of a controversial theory  

　　(D) question the methodology used in a study  

　　(E) discuss the implications of a research finding   

　　5. According to the passage, compared with Pleistocene carnivores in other areas, Pleistocene carnivores in the La Brea area (细节)

　　(A) included the same species, in approximately the same proportions  

　　(B) had a similar frequency of tooth fractures  

　　(C) populated the La Brea area more densely  

　　(D) consumed their prey more thoroughly  

　　(E) found it harder to obtain sufficient prey   

　　6. According to the passage, the researchers believe that the high frequency of tooth breakage in carnivores found at La Brea was caused primarily by (细节) (A) the aging process in individual carnivores  

　　(B) contact between the fossils in the pits  

　　(C) poor preservation of the fossils after they were removed from the pits  

　　(D) the impact of carnivores‟ teeth against the bones of their prey  

　　(E) the impact of carnivores’ teeth against the bones of other carnivores during fights over kills   

　　7. The researchers’ conclusion concerning the absence of demographic bias would be most seriously undermined if it were found that (逻辑) 

　　(A) the older an individual carnivore is, the more likely it is to have a large number of tooth fractures  

　　(B) the average age at death of a present-day carnivore is greater than was the average age at death of a Pleistocene carnivore  

　　(C) in Pleistocene carnivore species, older individuals consumed carcasses as thoroughly as did younger individuals  

　　(D) the methods used to determine animals‟ ages in fossil samples tend to misidentify many older individuals as younger individuals  

　　(E) data concerning the ages of fossil samples cannot provide reliable information about behavioral differences between extinct carnivores and present-day carnivores   

　　8. According to the passage, if the researchers had NOT found that two extinct carnivore species were free of tooth breakage, the researchers would have concluded that (推论) 

　　(A) the difference in breakage frequencies could have been the result of damage to the fossil remains in the La Brea pits  

　　(B) the fossils in other Pleistocene sites could have higher breakage frequencies than do the fossils in the La Brea pits  

　　(C) Pleistocene carnivore species probably behaved very similarly to one another with respect to consumption of carcasses  

　　(D) all Pleistocene carnivore species differed behaviorally from present-day carnivore species  

　　(E) predator densities during the Pleistocene era were extremely high

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