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 about cardiac 02 demand is \nSelect and explain. \nA. Directly propoional to mean aerial pressure, B. Inversely propoional to hea rate, C. Inversely propoional to cardiac work, D. Has a constant relation to the external work done by the hea', 'role': 'user'}, {'content': 'Answer: A \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'}]