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: Microbiology
[{'content': 'The factor which promotes virulence of M. tuberculosis - \nSelect and explain. \nA. Wax D, B. Cord factor, C. Muramyl dipeptide, D. Mycolic acid', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]