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': 'In which case spontaneus regression is not seen - \nSelect and explain. \nA. Malignant Melanoma, B. Osteosarcoma, C. Neuroblastoma, D. Choriocarcinoma', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans is 'b' ie Osteosarcoma", 'role': 'assistant'}]