This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous system, Subject name: Physiology
Which is not a peptide neurotransmitter? 
Select and explain. 
A. Enkephalin, B. Substance P, C. Endorphin, D. Serotonin
D: Serotonin
Answer: D: Serotonin 
Explanation: Small-Molecule, RapidlyActing TransmittersNeuropeptide, Slowly ActingTransmitters or Growth FactorsClass IAcetylcholineHypothalamic-Releasing HormonesThyrotropin-releasing hormoneLuteinizing hormone-releasing hormoneSomatostatin (growth hormone inhibitory factor)Class II: The AminesNorepinephrineEpinephrineDopamineSerotoninHistaminePituitary PeptidesAdrenocoicotropic hormoneb-Endorphina-Melanocyte-stimulating hormoneProlactinLuteinizing hormoneThyrotropinGrowth hormoneVasopressinOxytocinClass III: Amino AcidsGamma-aminobutyric acidGlycineGlutamateAspaatePeptides that Act on Gut and BrainLeucine enkephalinMethionine enkephalinSubstance PGastrinCholecystokininVasoactive intestinal polypeptideNerve growth factorBrain-derived neurotrophic factorNeurotensinInsulinGlucagonClass IVNitric oxideFrom Other TissuesAngiotensin IIBradykininCarnosineSleep peptidesCalcitoninRef: Guyton; 13th edition; Chapter 46 Organization of the Nervous System, Basic Functions of Synapses, and Neurotransmitters