PrepTest 146, Section 1, Question 9

What we’re looking for

Working the answers

• A
  Correct

  Here X is having more plant species, and Y is a better ability to support plant life. The argument takes their occurring together as evidence that X causes Y. But Y might cause X: better growing conditions could allow a wider range of species to thrive. A describes the exact alternative the argument overlooks.

• B

  The argument doesn't describe a mechanism, but that omission by itself doesn't identify the central problem. Perhaps varied plants help aerate the soil or otherwise improve growing conditions; we could investigate such a mechanism. The present evidence still leaves open whether better conditions produced the variety instead. A causal conclusion isn't automatically flawed just because the writer hasn't explained every physical step. Here the specific problem is the unaddressed direction of the relationship.

• C

  The study compares prairie plots; the passage doesn't describe drawing a conclusion from just one plot. More generally, using observations from studied plots to learn about other plots isn't automatically unreasonable. We would need a reason that the observations don't carry over. This choice doesn't address the direction of cause and effect that the reported pattern leaves unsettled.

• D

  What makes these data likely to be unrepresentative? We aren't given a particular selection problem or told that the studied plots are unusual. A study can represent prairies well and still leave this causal question unanswered. We don't need to assume a bad sample to explain why the argument falls short.

• E

  There's no move from a count to a percentage. The number here concerns how many species grow in a plot; the conclusion concerns the plot's ability to support plant life. The argument doesn't claim that a larger number must make up a larger share of some total.