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': 'Step down technique is \nSelect and explain. \nA. Preparation from apical to middle third, B. Preparation from apical to coronal third, C. Preparation from coronal to middle third, D. Preparation from coronal to apical third', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]