This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: A.N.S., Subject name: Pharmacology
[{'content': 'Which of the following is fastest acting receptor? \nSelect and explain. \nA. Beta1, B. M2, C. GABAa, D. Estrogen receptor', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. C. GABAaa. Ligand gated ion channels are the fastest acting receptors,b. Out of the options, the only ion channel receptor is GABAa.', 'role': 'assistant'}]