Which statement describes the condition of the heart at the point indicated in the electrocardiogram?
A. The atria are contracting and pushing blood into the bottom ventricles of the heart.
B. The ventricles are contracting and pushing blood out of the heart to the lungs and body.
C. The valves between the atria and ventricles close, producing the first heartbeat.
D. The valves of the aorta and pulmonary artery close, producing the first heartbeat.