This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Eye, Nose and Ear, Subject name: Anatomy
[{'content': 'The distance between tympanic membrane and medial wall of middle ear at centre is: \nSelect and explain. \nA. 3 mm, B. 4 mm, C. 6 mm, D. 2 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Middle ear cavity is shaped like a cube, compressed from side to side. In coronal section it resembles a biconcave disc. The veical and anteroposterior diameters are 15 mm each, and transverse diameters are: 6 mm (at roof), 2 mm (in the centre). 4 mm (at floor)', 'role': 'assistant'}]