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: Anatomy
[{'content': 'All are of adult size at birth except- \nSelect and explain. \nA. Mastoid antrum, B. Ear ossicles, C. Tympanic cavity, D. Maxillary antrum & orbit', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]