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': 'Cardiac output in 1./min divided by hea rate equals: \nSelect and explain. \nA. Cardiac efficiency, B. Mean stroke volume, C. Cardiac index, D. Mean aerial pressure', 'role': 'user'}, {'content': 'Answer: B \nExplanation: B i.e. Mean stroke volume', 'role': 'assistant'}]