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': "Billing's method of contraception refers to: \nSelect and explain. \nA. Monitoring BBT, B. Cervical mucus method, C. Rhythm method, D. Coitus interruptus method", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]