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: Unknown
[{'content': 'Establishment of fetoplacental circulation seen at \nSelect and explain. \nA. 11 to 13 days, B. 20 to 22 days, C. 7 days, D. 25 to 26 days', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ref. DC Dutta's Textbook of Obstetrics. Pg 36\n\xa0\nImplantation of 12-16 cell stage occurs in the endometrium on the anterior and posterior wall of the body near the fundus on the 6th day following fertilization.\n\nThen the deeper penetration of the human blastocyst is called interstitial implantation, which happens by approx. 13th day after fertilization\nThe fetal circulation is established by 21st days after fertilization", 'role': 'assistant'}]