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: Physiology
[{'content': 'Major portion of coronary blood flow occurs during: \nSelect and explain. \nA. Systole, B. Diastole, C. Not related to phases, D. It is variable', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]