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': 'While preparing the proximal box, the mesiodistal width of the ditch should be: \nSelect and explain. \nA. 0.8 mm ; two thirds in dentin and one third at the expense of enamel, B. 0.8 mm ; one third in dentin and two thirds at the expense of enamel, C. 0.8 mm ; full depth in dentin, D. 0.8 mm ; full depth in enamel', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Proximal Box\nIf the bur is allowed to cut only dentin, the resulting axial wall would be too deep. The mesiodistal width of the ditch should be 0.8 mm (the tip diameter of the bur) and prepared approximately two thirds (0.5 mm) at the expense of dentin and one third (0.3 mm) at the expense of enamel.\nSturdevant operative dentistry\xa07th Edition, Page No:e99', 'role': 'assistant'}]