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
[{'content': 'Single most important factor in control of automatic contractility of heart is \nSelect and explain. \nA. SA node pacemaker potential, B. Sympathetic stimulation, C. Right atrial volume, D. Myocardial wall thickness', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]