This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ear, Subject name: ENT
[{'content': 'Shortest length of middle ear: \nSelect and explain. \nA. 2 mm, B. 4 mm, C. 6 mm, D. 1 cm', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. (a) 2 mmRef: Dhingra's ENT 6th ed./ 9-10* Length of middle ear canal:# Epitympanum- 6mm# Mesotympanum- 2mm# Hypotympanum- 4mm* Shortest distance between tympanic membrane to promontory: 2mm.", 'role': 'assistant'}]