The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

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 （） to understand.