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': 'Opening of Eustachian tube in middle ear - \nSelect and explain. \nA. Medial wall, B. Lateral wall, C. Anterior wall, D. Posterior wall', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]