How do the semilunar valves prevent blood from propelling backwards?
a. They close tightly when the ventricles contract, preventing blood from flowing back into the atria.
b. They stay open when the ventricles contract, allowing blood to flow into the pulmonary artery and aorta.
c. They contract and relax in sync with the atria and ventricles, creating a one-way flow of blood.
d. They expand and contract to regulate the flow of blood from the atria to the ventricles.