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 gate control theory of pain, pain is modulated by: \nSelect and explain. \nA. A-delta fibres., B. Substantia gelatinosa., C. B fibres., D. C fibres.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]