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: Medicine
[{'content': 'S.A. node is pacemaker in heart because \nSelect and explain. \nA. It is only excitable, B. Its resting excitability is highest of all, C. It is only sensitive to vagus, D. It is the biggest pacemaker', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]