This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
[{'content': "McEwen's triangle is a landmark for \nSelect and explain. \nA. Antral drain, B. Mastoid surgery, C. Submucous resection, D. Uncinectomies", 'role': 'user'}, {'content': "Answer: B \nExplanation: Mac Ewen's (Suprameatal) Triangle: Boundaries of the triangle are: a. Anteriorly: posterosuperior border of the bony external auditory canal along with the spine of Henle. b. Posteriorly: a veical line drawn tangential to the posterior margin of the external, canal. c. Superiorly: the supramastoid crest. It is land mark for exposure of lateral surface of mastoid.", 'role': 'assistant'}]