Question ID:PT88 S2 Q3 Passage:Horses, although descended from a forest-dwelling ancestor, thrive in grasslands. In the last 2 million years, horses have gone through three cycles of population increase followed by a rapid decline in population. The most recent cycle peaked 25,000 years ago during a cold period that preceded a period of warming. Stem:Which one of the following, if true, most helps to explain why the horse population peaked 25,000 years ago and then rapidly declined?  Correct Answer Choice:DChoice A:The forest-dwelling ancestor of horses was many times smaller than horses. Choice B:The only true wild horse existing today, Przewalski's horse, inhabits the cold grasslands of Central Asia. Choice C:Some modern species that are related to horses, such as zebras, inhabit the warm grasslands of Africa. Choice D:During cold periods there are extensive grasslands, but these tend to revert to forest when the climate warms. Choice E:Several cycles of a cold period followed by a period of warming have occurred in the last 2 million years.