Which of the Following Would Be an Example of the Term

Which of the following would be an example of the term cardiac reserve?

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 having completed an exercise routine
D) at the end of systole, approximately 50 mL of blood remains in the ventricles