填空题 Passage Two Why is our appetite so powerful a driver of our behavior? If that question has long defied easy answers, it’s no wonder. Understanding a single biological unit — the heart, the lungs — is hard enough. Understanding a process as complex as appetite — one that involves taste, smell, sight, brain chemistry, gut chemistry, metabolism and, most confounding of all, psychology — is exponentially harder. Appetite mechanism is d ---|||________|||--- to understand.