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
Severity if mitral stenosis is judged by- 
Select and explain. 
A. Loud S1, B. S2-OS gap, C. S3, D. S4
B: S2-OS gap
Answer: B: S2-OS gap 
Explanation: None