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: Pharmacology
[{'content': 'Prophylactic antibiotics to prevent surgical site infection are best administered : \nSelect and explain. \nA. After commencement of surgery, B. 30 minutes before incision, C. At the end of surgery, D. With pre medication', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]