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': 'Which sinus is the last sinus to appear radiologically on X-ray? \nSelect and explain. \nA. Maxillary sinus, B. Sphenoid sinus, C. Frontal sinus, D. Ethmoidal air cells', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans. is'c'i.e., Frontal sinus[Ref Textbook Of The Ear, Nose And Throat By De Soum, C. Et Al. p 394)", 'role': 'assistant'}]