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': 'If both the ala of the sacrum are absent, Charecteristic of \nSelect and explain. \nA. Naegele pelvis, B. Robert pelvis, C. Triradiate pelvis, D. Rachitic pelvis', 'role': 'user'}, {'content': "Answer: B \nExplanation: A pelvis that is narrowed transversely due to the entire\xa0absence of both the alae of sacrum\xa0is charecteric of Robert's pelvis", 'role': 'assistant'}]