In paragraph 4, why does the author emphasize the amount of effort and resources needed to grow alfalfa and oats? A. To illustrate how valuable horses were by showing how much farmers were willing to sacrifice to keep them B. To provide evidence that, in medieval Europe, both horses and humans lived at the margins of a sufficient diet C. To argue that it made more sense to devote land to growing food than food for horses D. To explain why oxen and other cattle that ate grass and hay continued to be more common than horses