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': 'Severity if mitral stenosis is judged by- \nSelect and explain. \nA. Loud S1, B. S2-OS gap, C. S3, D. S4', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]