This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
[{'content': 'True regarding myocardial 02 demand \nSelect and explain. \nA. Inversely related to hea rate, B. Has constant relation to external cardiac work, C. Directly propoional to duration of systole, D. Is negligible at rest', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Experimental studies have shown that oxygen consumption of the hea and the chemical energy expended during contraction are directly related to the external work Ref: Guyton and Hall Textbook of Medical Physiology (12th Edn) page No:209', 'role': 'assistant'}]