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': 'The best investigation for dating pregnancy Is : \nSelect and explain. \nA. Crown rump length at 9 weeks, B. BPD at 18 weeks, C. Abdominal circumference at 24 weeks, D. BPD at 24 weeks', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Crown rump length at 9 weeks', 'role': 'assistant'}]