This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Skeletal system, Subject name: Radiology
[{'content': "Onion - peel appearance is seen in - \nSelect and explain. \nA. Ewing's sarcoma, B. Osteoma, C. Osteosarcoma, D. Osteoclastoma", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Onion peel periosteal reaction is seen in Ewings sarcoma.', 'role': 'assistant'}]