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': 'All of the follwoing are of the size of adult at bih expect? \nSelect and explain. \nA. Tympanic membrane, B. Ossicle, C. Tympanic cavity, D. Mastoid', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Development of the mastoid air cell system does not occur until afterbih, with about 90% of air cell formation being completed by the age of six with the remaining 10% taking place up to age of 18', 'role': 'assistant'}]