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': 'Retention in a stainless steel crown is achieved primarily by: \nSelect and explain. \nA. Chamfer, B. Cement, C. Parallel distal and mesial walls, D. Crimping the crown to engage buccal & lingual undercuts', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]