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': 'The first heart sound is produced by the: \nSelect and explain. \nA. Closure of the aortic and pulmonary valves, B. Opening of the aortic and pulmonary valves, C. Closure of the mitral and tricuspid valves, D. Opening of the mitral and tricuspid valves', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]