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
Retention in a stainless steel crown is achieved primarily by: 
Select and explain. 
A. Chamfer, B. Cement, C. Parallel distal and mesial walls, D. Crimping the crown to engage buccal & lingual undercuts
D: Crimping the crown to engage buccal & lingual undercuts
Answer: D: Crimping the crown to engage buccal & lingual undercuts 
Explanation: None