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
True about cardiac 02 demand is 
Select and explain. 
A. 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
A: Directly propoional to mean aerial pressure
Answer: A: Directly propoional to mean aerial pressure 
Explanation: 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