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': 'All enamel walls of a cavity preparation must consist of full length enamel rods on \nSelect and explain. \nA. Affected enamel, B. Affected dentin, C. Sound enamel, D. Sound dentin', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]