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': 'A ledge is created on the lingual surface of the full\ncast crown, to accommodate the lingual clasp arm. The greatest advantage is that it provides for: \nSelect and explain. \nA. Appropriate contact, B. Appropriate undercut, C. Survey line, D. Guiding plane', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]