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': "Principle of retainer placement is/are: \nSelect and explain. \nA. Bow is always placed on distal surface of tooth., B. Prong should have 4 point contact with tooth., C. Jaw shouldn't extent beyond mesial and distal axial line angles., D. All of the above.", 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]