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
Which of the following statements about myocardial oxygen demand is true 
Select and explain. 
A. Correlates with heart rate, B. Is directly proportional to external cardiac work, C. Is negligible when heart is at rest, D. Depends upon duration of systole
D: Depends upon duration of systole
Answer: D: Depends upon duration of systole 
Explanation: None