This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fertilization and Development of Embryo, Subject name: Anatomy
[{'content': 'Three layers of embryo are formed at what age? \nSelect and explain. \nA. 8 days, B. 12 days, C. 16 days, D. 21 days', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. (d) 21 daysRef: Langmanris embryology 10th ed ch:5', 'role': 'assistant'}]