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': 'Clamshell technique is required for: \nSelect and explain. \nA. llio-cortico graft, B. llio-cancellous graft, C. Posterior iliac graft, D. Anterior iliac graft', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]