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': 'First heart sound corresponds to: \nSelect and explain. \nA. Mitral valve opening, B. Mitral valve closing, C. Aortic valve closing, D. None of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The first heart sound (S1) includes mitral and tricuspid valve closure.\xa0\n\nThe First heart sound is associated with closure of the A-V Valves.\nThe Second heart sound Is associated with closure of the Aortic and Pulmonary Valves.\nThe\xa0Third\xa0heart\xa0sound occurs\xa0at\xa0the\xa0beginning\xa0of the middle third\xa0of diastole. Occasionally a weak, rumbling third heart sound is heard at the beginning of the middle third of diastole.\xa0\xa0\nAtrial contraction sound (Fourth heart sound).\xa0\n\nReference: : Guyton and Hall Physiology page no\xa0 283,284', 'role': 'assistant'}]