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': 'Trophoblast differentiates at _ post-fertilization day? \nSelect and explain. \nA. 8-Jun, B. 12-Aug, C. 14-Dec, D. 16-18', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. a. 6-8 Explanation: Trophoblast differentiates into cyto and syncytiotrophoblast at 6 - 8th-day post ovulation/fertilization. The references are from Gray's anatomy, Langman Embryology, and Moore Embryology.", 'role': 'assistant'}]