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': 'How many days after ovulation is placental circulation established : \nSelect and explain. \nA. 18-21, B. 11-Sep, C. 9-Jul, D. 30-33', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 18-21', 'role': 'assistant'}]