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': 'In step back technique, the file used for measuring working length is 25 no and it is the first file to be used for Phase 2 i.e. step back technique, the file used for recapitulation will be \nSelect and explain. \nA. No 10, B. No 15, C. No 20, D. No 25', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]