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
Which sinus is the last sinus to appear radiologically on X-ray? 
Select and explain. 
A. Maxillary sinus, B. Sphenoid sinus, C. Frontal sinus, D. Ethmoidal air cells
C: Frontal sinus
Answer: C: Frontal sinus 
Explanation: Ans. is'c'i.e., Frontal sinus[Ref Textbook Of The Ear, Nose And Throat By De Soum, C. Et Al. p 394)