This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which is not a peptide hormone? 
Select and explain. 
A. Somatostatin, B. Serotonin, C. Neuropeptide, D. Enkephalin
B: Serotonin
Answer: B: Serotonin 
Explanation: Somatostatin is a polypeptide hormone.