This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
[{'content': 'When does the Implantation occur? \nSelect and explain. \nA. On day 4 after feilization, B. After the endometrium undergoes a decidual reaction, C. When the embryo is at the morula stage, D. Only after priming of the uterine endometrium by progesterone and estrogen', 'role': 'user'}, {'content': 'Answer: D \nExplanation: .', 'role': 'assistant'}]