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': 'Sinus which is not present at bih is? \nSelect and explain. \nA. Maxillary sinus, B. Ethmoid sinus, C. Frontal sinus, D. All of the above', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans. is'c'i.e., Frontal sinus", 'role': 'assistant'}]