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': "Total cardiac output doesn't change during \nSelect and explain. \nA. Sleep, B. From supine to standing position, C. Exercise, D. Arrhythmias", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Sleep does not effect cardiac output.', 'role': 'assistant'}]