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 one is excitatory neurotransmitter? 
Select and explain. 
A. Acetylcholine, B. Histamine, C. Melatonin, D. GABA
A: Acetylcholine
Answer: A: Acetylcholine 
Explanation: acetylcholine serves excitatory and inhibitory functions, which means that ACh can speed up or slow down nerve signals. In the central nervous system, its function is mainly excitatory Ref : guyton and hall textbook of medical physiology 12 edition page number:677,678,679