This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
True about cardiac O2 demand is- 
Select and explain. 
A. Directly proportional to mean arterial pressure, B. Inversely proportional to heart rate, C. Inversely proportional to cardiac work, D. Has a constant relation to the external work done by the heart
A: Directly proportional to mean arterial pressure
Answer: A: Directly proportional to mean arterial pressure 
Explanation: Ans. (a) Directly proportional to mean arterial pressureRef: Ganong, 25th ed/p.612Myocardial oxygen consumption is directly proportional to mean arterial pressure