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
In cavity preparation, line angle is 
Select and explain. 
A. 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
C: The junction of two plane surfaces of different orientation along a line
Answer: C: The junction of two plane surfaces of different orientation along a line 
Explanation: None