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': 'All of the following statements about myocardial oxygen demand is true, except \nSelect and explain. \nA. Correlates with heart rate, B. Is proportional to external cardiac work, C. Is negligible when heart is at rest, D. Depends upon duration of systole', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Oxygen demand depends on the work being done. The work done is to pump the blood which is in the ventricles (preload) against the resistance in the aorta (afterload).\n\xa0So, myocardial oxygen demand depends on\n\nRate – Directly proportional\nContractility –\xa0Directly proportional\nIntramyocardial tension –\xa0Directly proportional\nAfter load –\xa0Directly proportional\nPreload –\xa0Directly proportional\nWall thickness –\xa0Inversely proportional\n\nThe oxygen demand of myocardium is not having the same change with varying external works. But it is proportional to the external work.', 'role': 'assistant'}]