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': 'In cavity preparation, line angle is \nSelect and explain. \nA. Union of three surfaces, B. Union of four surfaces, C. The junction of two plane surfaces of different orientation along a line, D. Internal boundary of a cavity', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]