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': "Which of the following is the investigation of choice in a pregnant lady at 18 weeks of pregnancy with pasthistory of delivering a baby with Down's syndrome: \nSelect and explain. \nA. Triple screen test, B. Amniocentesis, C. Chorionic villous biopsy, D. Ultrasonography", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]