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: Dental
[{'content': 'The prognosis of the broken instrument in root canal is good when \nSelect and explain. \nA. Instrument is less than 20 nos., B. Instrument is lodged in the bone beyond the apical area, C. Instrument obliterates apical third of the canal, D. Instrument is in middle', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]