This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Head and neck, Subject name: Anatomy
All are of adult size at bih except 
Select and explain. 
A. Mastoid antrum, B. Ear Ossicles, C. Tympanic cavity, D. Maxillary antrum
D: Maxillary antrum
Answer: D: Maxillary antrum 
Explanation: The paranasal sinuses are hollow, air-filled cavities that are lined by a mucous membrane. The ethmoid and maxillary sinuses are present at bih. The frontal sinus develops about the second year of life and the sphenoid about the third year. Sometimes, the frontal sinuses do not develop.