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: Surgery
[{'content': 'In Caldwell-Luc operation, opening to the sinus is made? \nSelect and explain. \nA. At the premolar region, B. At the incisor region, C. At the 1st molar region, D. At the 2nd molar region', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]