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': 'What is the best prognostic indicator of multiple myeloma at the time of diagnosis - \nSelect and explain. \nA. Number of myeloma cells in the marrow, B. Beta 2 microglobulin, C. Alkaline phosphatase level, D. Hypercalcemia', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]