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
Sequestrum means: 
Select and explain. 
A. 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
A: It is necrosed bone, separated from its surrounding
Answer: A: It is necrosed bone, separated from its surrounding 
Explanation: None