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: Gynaecology & Obstetrics
[{'content': 'Which natural family planning method is based on ogino knauss theory: \nSelect and explain. \nA. BBT method, B. Rhythm method, C. Lactational anerorrhea, D. Withdrawl method', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]