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
Which of the following impart structural durability in a three quarter crown  preparation on a mandibular molar? 
Select and explain. 
A. Occlusal shoulder, B. Functional cusp bevel, C. Proximal groove, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None