The Nurse Is Explaining Cardiac Reserve to a Patient Recovering

The nurse is explaining cardiac reserve to a patient recovering from a myocardial infarction. Which example should the nurse use to explain this term?

A) getting on a treadmill and gradually increasing the pace of walking
B) breathing in through the nose and out the mouth while sitting quietly
C) sitting in a chair to cool down after completing an exercise routine
D) at the end of systole, approximately 50 mL of blood remaining in the ventricles