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': "The single most powerful predictor of survival in multiple myeloma is - \nSelect and explain. \nA. `M' component production, B. Bone marrwo plasmocytosis, C. Serum beta 2-microglobulin level, D. Serum calcium level", 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]