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': 'Loud S1 in mitral stenosis is seen in \nSelect and explain. \nA. Prolonged flow through mitral valve, B. 1st degree heart block, C. Calcification of the valve, D. Immobilization of the valve', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]