The author of the passage mentions accounts from three white frontiersmen primarily in order to A. show how the frontiersmen's dietary choices were influenced by the native groups with whom they came into contact. B. suggest that these frontiersmen had not adapted well to a diet composed primarily of lean meat. C. indicate what kind of diet was habitually followed by native big-game hunters of the Northern Plains. D. identify evidence for a hypothesis regarding native hunters' fish avoidance. E. call into question an assumption about the effects of fish consumption on people who have adapted to a lean-meat diet.