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
Principle of retainer placement is/are: 
Select and explain. 
A. 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.
D: All of the above.
Answer: D: All of the above. 
Explanation: None