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: Unknown
[{'content': "Tumour arising from diaphysis - \nSelect and explain. \nA. Osteogenic sarcoma, B. Ewing's sarcoma, C. Osteoclastoma, D. Osteochondroma", 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans-B', 'role': 'assistant'}]