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': 'Least likely to regress spontaneously is \nSelect and explain. \nA. Osteosarcoma, B. Retinoblasoma, C. Choriocarcinoma, D. Malignant melanoma', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Osteosarcoma", 'role': 'assistant'}]