Question ID:PT46 S1 Q17 Passage:The pronghorn, an antelope-like mammal that lives on the western plains of North America, is the continent's fastest land animal, capable of running 90 kilometers per hour and of doing so for several kilometers. Because no North American predator is nearly fast enough to chase it down, biologists have had difficulty explaining why the pronghorn developed its running prowess. One biologist, however, has recently claimed that pronghorns run as fast as they do because of adaptation to predators known from fossil records to have been extinct for 10,000 years, such as American cheetahs and long-legged hyenas, either of which, it is believed, were fast enough to run down the pronghorn. Like all explanations that posit what is called a relict behavior‚ a behavior that persists though its only evolutionary impetus comes from long-extinct environmental conditions‚ this one is likely to meet with skepticism. Most biologists distrust explanations positing relict behaviors, in part because testing these hypotheses is so difficult due to the extinction of a principal component. They typically consider such historical explanations only when a lack of alternatives forces them to do so. But present-day observations sometimes yield evidence that supports relict behavior hypotheses.In the case of the pronghorn, researchers have identified much supporting evidence, as several aspects of pronghorn behavior appear to have been shaped by enemies that no longer exist. For example, pronghorns‚ like many other grazing animals‚ roam in herds, which allows more eyes to watch for predators and diminishes the chances of any particular animal being attacked but can also result in overcrowding and increased competition for food. But, since pronghorns have nothing to fear from present-day carnivores and thus have nothing to gain from herding, their herding behavior appears to be another adaptation to extinct threats. Similarly, if speed and endurance were once essential to survival, researchers would expect pronghorns to choose mates based on these athletic abilities, which they do‚ with female pronghorns, for example, choosing the victor after male pronghorns challenge each other in sprints and chases.Relict behaviors appear to occur in other animals as well, increasing the general plausibility of such a theory. For example, one study reports relict behavior in stickleback fish belonging to populations that have long been free of a dangerous predator, the sculpin. In the study, when presented with sculpin, these stickleback fish immediately engaged in stereotypical antisculpin behavior, avoiding its mouth and swimming behind to bite it. Another study found that ground squirrels from populations that have been free from snakes for 70,000 to 300,000 years still clearly recognize rattlesnakes, displaying stereotypical antirattlesnake behavior in the presence of the snake. Such fear, however, apparently does not persist interminably. Arctic ground squirrels, free of snakes for about 3 million years, appear to be unable to recognize the threat of a rattlesnake, exhibiting only disorganized caution even after being bitten repeatedly. Stem:Based on the passage, the term "principal component" (second sentence of the second paragraph) most clearly refers to which one of the following? Correct Answer Choice:DChoice A:behavior that persists even though the conditions that provided its evolutionary impetus are extinct Choice B:the original organism whose descendants' behavior is being investigated as relict behavior Choice C:the pronghorn's ability to run 90 kilometers per hour over long distances Choice D:the environmental conditions in response to which relict behaviors are thought to have developed Choice E:an original behavior of an animal of which certain present-day behaviors are thought to be modifications