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
The distance between tympanic membrane and medial wall of middle ear at the level of center is - 
Select and explain. 
A. 3mm, B. 4 mm, C. 6mm, D. 2 mm
D: 2 mm
Answer: D: 2 mm 
Explanation: None