【正确答案】 A

【答案解析】 Argument Construction
   Situation  Black ants help to control populations of rodents by competing with them for seeds. But a very
              aggressive species of black ant, the Loma ant, has a sting that can be fatal to humans. Ant flies
              prey on Loma ants and their presence can thereby benefit public health.
   Reasoning  Which of the possible completions of the passage provides the most support for the conclusion? The
              argument's conclusion is that introducing ant flies into the region where Loma ants have recently
              invaded would benefit public health. We know from the passage that black ants, generally, benefit
              public health by keeping down rodent populations. However, the sting of Loma ants, a species of
              black ant, can be fatal to humans. Ant flies prey on Loma ants. To that extent their introduction
              in the region would tend to benefit public health by making fatal Loma stinging of humans less
              likely. But if these ant flies also prey on black ants other than the Loma ants, then to that extent
              they would undermine another public health benefit associated with controlling rodents. Thus, the
              information that ant flies do not prey on black ants other than Loma ants would provide strong
              logical support for the conclusion.
   A   Correct. This most logically completes the argument because it addresses a potential downside of introducing the ant flies into the region. The potential downside is that it might reduce the desirable effect that other species of black ants have in keeping down the rodent populations.
   B   We have no idea whether the bird species that are more effective than Loma ants at competing with rodents for seeds are even present in the region in question.
   C   This does not help the conclusion very much because we do not know from the passage whether ant flies prey on other species of black ants besides Loma ants.
   D   If anything, this is a reason not to introduce ant flies into the region. This answer choice at least suggests that Loma ants might have some positive effect on public health because they might keep down rodent populations by reducing their survival chances.
   E   This provides very little support for the conclusion. It does not exclude the possibility that there are pesticides—perhaps several—that would control Loma ants effectively without harming the environment. So it is not a strong reason for introducing ant flies.
   The correct answer is A.