4. Which of the following statements are accurate?
A. The heartbeat in a human heart is initiated by the atrioventricular mode.
B. During systole, the heart's ventricle(s) is/are relaxing and filling with blood.
C. During systole, blood will be pumped from the heart into the veins of the body
D. The pulse is used to determine one's heart rate.
E. The nervous system and certain hormones can alter pacemaker activity.