This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Zona hatching occurs \nSelect and explain. \nA. 4 days after feilisation, B. 5 days after feilisation, C. 6 days after feilisation, D. 8 days after feilisation', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Five days after feilisation, blastocyst performs zona hatching and it undergoes degeneration to be replaced by trophoblastic cells', 'role': 'assistant'}]