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: Social & Preventive Medicine
[{'content': "What is Grafenbers's Ring \nSelect and explain. \nA. Vaginal Pessary to prevent prolapse of uterus, B. I Generation IUD, C. A mechanical barrier to prevent pregnancy, D. None of the above", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]