Arteries are strong, elastic vessels that carry blood to the heart.

This is because anatomy and physiology are closely related branches of biology that study the structure and function of living organisms respectively.
Anatomy describes the shape, size, location, and relationships of body parts, while physiology explains how those parts work together to maintain life.
For example, the anatomy of the heart determines how it pumps blood, and the anatomy of the lungs determines how they exchange gases.
Choice B is wrong because our understanding of both anatomy and physiology is constantly changing as new discoveries are made in the field of biology.
Choice C is wrong because body parts take up space regardless of their physiological functions.
Choice D is wrong because physiological functions are not limited to an organism, but can also occur at the cellular, tissue, organ, and system levels.

The sinoatrial (SA) node is the natural pacemaker of the heart.
It generates electrical impulses that cause the heart’s chambers to contract, setting the heart rate.
The SA node typically generates 60–100 heartbeats per minute when a person is at rest.
Choice A is wrong because Purkinje fibers are part of the electrical conduction system of the heart that carries impulses to the ventricles.
They are not the primary source of impulses.
Choice B is wrong because the AV bundle (also called bundle of His) is another part of the electrical conduction system of the heart that connects the atria and the ventricles.
It is not the natural pacemaker of the heart.
Choice D is wrong because an atrioventricular node is a relay station between the atria and the ventricles that delays the electrical impulses from the SA node.
It can act as a backup pacemaker if the SA node fails, but it is not the normal pacemaker of the heart.