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': 'Most conservative method of root end preparation is by: \nSelect and explain. \nA. Round bur., B. Sonic., C. Ultrasonic., D. Laser.', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]