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': 'Sequestrum means: \nSelect and explain. \nA. It is necrosed bone, separated from its surrounding, B. It is a broken tooth piece, C. It is new bone formed in osteomyelitis, D. It is necrosed tooth', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]