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': 'Normal crown-rump length at bih is : \nSelect and explain. \nA. 20-25 cms., B. 38-50 cms, C. 80-72 cms, D. 78-85 cms', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 38-50 cms', 'role': 'assistant'}]