Why does the author mention "old-fashioned steam radiator" in the discussion of countercurrent exchange systems? A. To argue that a turtle's central heating system is not as highly evolved as that of other warmblooded animals B. To provide a useful comparison with which to illustrate how a countercurrent exchange system works C. To suggest that steam radiators were modeled after the sophisticated heating system of turtles D. To establish the importance of the movement of water in countercurrent exchange systems