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: Orthopaedics
[{'content': "A most common cause of bone malignancy- \nSelect and explain. \nA. Secondaries, B. Osteosarcoma, C. Ewing's sarcoma, D. Osteoclastoma", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Metastatic bone disease is the commonest malignancy of bones and is much more common than primary bone tumours', 'role': 'assistant'}]