The author implies which of the following about the "underlying body parts" of nonhuman mammals? A. They are more similar to humans' underlying body parts than most people assume. B. They are more predictable in their workings than are the underlying body parts of humans. C. Their smooth and grateful actions belie how complicated they actually are. D. They work in much different ways than do the underlying body parts of most birds. E. They are easier to observe and understand than are underlying parts of birds.