PrepTest 155, Section 2, Question 9
Rodents are small, gnawing mammals characterized by their(23 more words not shown)
Which one of the following(8 more words not shown)
- AMost species of(7 more words not shown)
- BIn North America,(14 more words not shown)
- CMost species of(12 more words not shown)
- DOf the mammal(16 more words not shown)
- EMost nonrodent mammal(7 more words not shown)
Reading the argument
Count the kinds of mammals separately from the individual animals. Rodents make up fewer than half of North America's mammal species, yet they account for more than half of its individual mammals. In other words, the smaller collection of species contains the larger collection of animals. Those rodent species must therefore have more members per species on average.
Imagine a few rodent species with huge numbers of rats and mice, alongside many other mammal species whose populations are smaller. That would make the two facts fit together. Having more kinds of nonrodents doesn't mean having more nonrodent animals.
What we’re looking for
For the most-supported question, that's our prediction: rodent species tend to have more individual members than other mammal species. Keep that at the level of the overall comparison. It doesn't automatically tell us which single species has the largest population.
Working the answers
- A
Rodents have these teeth, but most mammal species in North America are not rodents. The fact that rodents make up most individual mammals doesn't turn them into most species. We also aren't told enough about nonrodents' teeth to establish A by some other route. The answer mixes up the two kinds of counts.
- BCorrect
Fewer rodent species account for more individual animals. That requires a higher average number of individuals per rodent species than per nonrodent mammal species. B expresses this as a tendency. It doesn't claim that every rodent species outnumbers every other mammal species.
- C
We know something about mammals within North America. We haven't compared them with mammals in Africa, South America, Australia, or anywhere else. A description of North America's rodents doesn't establish that most species with these teeth live there.
- D
A nonrodent species could be the single largest even while rodents have the higher average. For example, imagine two rodent species with 40 animals each and three nonrodent species with 50, 5, and 5. Rodents have fewer species and more animals overall—80 versus 60—but the largest individual species is a nonrodent with 50. The given facts therefore don't establish D.
- E
Again, where are most of the world's species found? The passage never answers that. Saying most mammal species within North America are nonrodents doesn't mean most nonrodent mammal species in the world can be found in North America. Those statements compare different groups.
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