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
Sinus which is not present at bih is? 
Select and explain. 
A. Maxillary sinus, B. Ethmoid sinus, C. Frontal sinus, D. All of the above
C: Frontal sinus
Answer: C: Frontal sinus 
Explanation: Ans. is'c'i.e., Frontal sinus