The answer is microscopic life appeared on Earth before flowering plants. This is because according to the label on the second box, microscopic life appeared 4.0 to 3.5 billion years ago, and this is way before the date 250 to 120 millions years ago indicated on the label on the sixth box the flowering plants appeared on Earth.
Because the AV node takes on the role of a very important delay in the cardiac conduction system. Without this delay, the atria and ventricles would contract at the same time.
<h3>Cardiac conduction system</h3>
The conduction system of the heart sends electrical signals, usually generated by the sinus nodes, causing the heart muscle to contract. Pacemaker signals originating from the sinus node travel through the right atrium to the atrioventricular (AV) along the bundle of His and through branches of the bundle, causing myocardial contraction. This signal first stimulates contraction of the right and left atria then flows to the right and left ventricles. That electric current then allows blood to be pumped throughout the body.
Learn more about sequence of travel by an impulse through the cardiac conduction system at brainly.com/question/28269433
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The answer is:
B. Cell Cycle
