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: Social & Preventive Medicine
The strain which is used for production of BCG vaccine at commercial level is 
Select and explain. 
A. Danish-1331, B. Tween-80, C. Bacille Calmette Guerin, D. PPD-RT-23
A: Danish-1331
Answer: A: Danish-1331 
Explanation: None