The correct answer is C. Valve.
The heart has four valves: the mitral valve, the tricuspid valve, the aortic valve, and the pulmonic valve. These valves open and close to allow blood to flow through the heart in the correct direction.
The mitral valve and the tricuspid valve are located between the atria and the ventricles. The aortic valve and the pulmonic valve are located between the ventricles and the arteries.
When the ventricles contract, the valves close to prevent blood from flowing back into the atria. This closure of the valves produces the first heart sound, which is a lub sound.
When the ventricles relax, the valves open to allow blood to flow into the arteries. This opening of the valves produces the second heart sound, which is a dub sound.
The audible sounds of the heart are important because they can be used to assess the health of the heart. For example, a heart murmur is an abnormal sound that can be caused by a problem with one of the valves.