Official Explanation

Argument Construction

This question asks us to find an assumption that allows the biologist's conclusion to be logically drawn if made in conjunction with the premises of the biologist's argument.

The biologist claims that species with broader ranges are more likely to survive the extinction of populations in certain areas than are species with narrow ranges. The biologist concludes that over time the proportion of species with broader ranges will probably increase.

Note that the argument as it stands is not logically valid: it is possible that if species that now have broad ranges survive the extinction of populations with narrow ranges, the proportion of species with broad ranges could still decline. That is, decimation of populations in certain areas may in fact cause the ranges of species that now have broad ranges to shrink in size, thereby becoming narrow ranges.

Were this to happen at a faster pace than the extinction of species that currently have narrow ranges, the proportion of species with broad ranges would decline rather than increase. The correct answer to this question must rule out this possibility.

A. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline.

B. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline.

C. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline. In fact, this assumption helps to support the claim that as certain populations of a species that once had a broad range die out, that species' range could narrow.

D. Correct. This assumption rules out the possibility described above; furthermore, it rules out any other possibility that allows the biologist's conclusion to be false even if the premises were true.

E. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline.

The correct answer is D.