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': 'Caldwell Iuc approach opens - \nSelect and explain. \nA. Maxillary sinus, B. Frontal sinus, C. Sphenoid sinus, D. Ethmoid sinus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]