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
The distance between tympanic membrane and medial wall of middle ear at centre is: 
Select and explain. 
A. 3 mm, B. 4 mm, C. 6 mm, D. 2 mm
D: 2 mm
Answer: D: 2 mm 
Explanation: 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)