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
What is the best prognostic indicator of multiple myeloma at the time of diagnosis - 
Select and explain. 
A. Number of myeloma cells in the marrow, B. Beta 2 microglobulin, C. Alkaline phosphatase level, D. Hypercalcemia
B: Beta 2 microglobulin
Answer: B: Beta 2 microglobulin 
Explanation: None