This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nose and paranasal sinuses, Subject name: ENT
[{'content': 'Sinus not present at bih is: \nSelect and explain. \nA. Ethmoid, B. Maxillary, C. Sphenoid, D. Frontal', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Frontal-Absent at bih-Adult size in20 years-Invades frontal bone(2-4yrs),size increases until teens. Ref.Scott Brown 7/e,Vol 2,p 1320.', 'role': 'assistant'}]