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: Surgery
[{'content': "Most common primary malignant tumor of long bones is: \nSelect and explain. \nA. Osteosarcoma, B. Multiple Myeloma, C. Ewing's sarcoma, D. Metastatic carcinoma", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Excluding multiple myeloma and lymphoma, osteosarcoma is the most common primary tumor of bone and accounts for 20-36% of primary bone malignancies.', 'role': 'assistant'}]