This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CVS Embryology, Subject name: Anatomy
[{'content': 'Hea begins to beat in the week: \nSelect and explain. \nA. 4, B. 5, C. 6, D. 7', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Hea beat begins by day 22 post-ovulation and can be detected by Doppler ultrasound. It is week 4 post-ovulation (or feilization) and 6 weeks from LMP (Last Menstrual Period).', 'role': 'assistant'}]