Explanation
Correct answer: C.
Choice C is the best answer because it most logically completes the text’s discussion of the evolution of mammalian brain-to-body size ratios. As the text explains, mammals evolved to fill ecological niches left vacant by the dinosaurs’ extinction, and for the next ten million years, brain-to-body size ratios among mammals shrank as their bodies grew more rapidly than their brains. This decline was a natural outcome of resource abundance since increased competition for resources tends to favor intelligence and large brains, while decreased competition conversely favors large body size. Later, mammalian population sizes “swelled,” resulting in intensified competition—that is, a decrease in the abundance of resources relative to population sizes. Given the relationship established between resource availability and the ratio of brain size to body size, it can be inferred that around ten million years after the dinosaurs’ extinction, ecological conditions favored an evolutionary increase in brain size relative to body size among mammals—a reversal of the previous trend.
Why the other choices are wrong
Choice A
Choice A is incorrect because, as the text explains, competition for resources tends to favor intelligence and large brains, not large body size. Thus, when mammalian population sizes swelled and competition among mammals intensified, the result would have been a decrease, not an increase, in the advantage that large body size conferred on mammals in certain ecological niches.
Choice B
Choice B is incorrect because the text frames competition for resources as a selective pressure favoring intelligence and brain size relative to body size. Thus, even if competition among mammals resulted in resource deprivation, these conditions would have favored adaptation toward larger brain-to-body size and consequently greater intelligence to compete more effectively for decreased resources.
Choice D
Choice D is incorrect. The period when mammals adapted to a range of ecological niches that is under discussion in the text is the ten million years immediately following the dinosaurs’ extinction, when competition for resources was low. Moreover, the text explains that increased competition tends to favor brain size, while decreased competition tends to favor body size. Thus, when competition increased, the logical outcome would have been an increase in brain size relative to body size among mammals, not a dispersal of large-brained mammals into new ecological niches, which the text states had already occurred under the very different conditions of low competition.