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
Resistance form of cavity preparation is: 
Select and explain. 
A. Flat pulp floor, B. enough depth of the cavity, C. Restricted size of the external walls, D. all of the above
D: all of the above
Answer: D: all of the above 
Explanation: None