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: ENT
[{'content': "All of the following form the boundary of MacEwen's triangle except: \nSelect and explain. \nA. Temporal line, B. Posterosuperior segment of bony external auditory canal, C. Promontory, D. Tangent drawn to the external auditory meatus", 'role': 'user'}, {'content': "Answer: C \nExplanation: Mastoid antrum is marked externally on the surface by suprameatal (Mac Ewen's) triangle.", 'role': 'assistant'}]