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: Dental
[{'content': 'When P.D. probing in gingivitis done, depth is measured\nfrom \nSelect and explain. \nA. Base of pocket to CD, B. Base of pocket to free marginal gingiva, C. Marginal gingiva to CEJ, D. Junctional epithelium to free gingival margin', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]