14. Many endotherms use a countercurrent exchange system to conserve heat. How does this function? (A) Heat is transferred from arteries to veins against a thermal gradient. (B) Heat is transferred from veins to arteries against a thermal gradient. (C) Heat is transferred from arteries to veins along a thermal gradient. (D) Heat is transferred from veins to arteries along a thermal gradient. (E) Heat is transferred either from-veins-to-arteries or from-arteries-to-veins, as long as it is along a thermal gradient.