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: Anatomy
[{'content': 'Implantation occurs at the stage of - \nSelect and explain. \nA. Zygote, B. Morula, C. Blastocyst, D. Primary villi', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]