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
Most common primary malignant tumor of long bones is: 
Select and explain. 
A. Osteosarcoma, B. Multiple Myeloma, C. Ewing's sarcoma, D. Metastatic carcinoma
A: Osteosarcoma
Answer: A: Osteosarcoma 
Explanation: Excluding multiple myeloma and lymphoma, osteosarcoma is the most common primary tumor of bone and accounts for 20-36% of primary bone malignancies.