Introduction:
The semilunar valves are located between the ventricles and the major blood vessels leaving the heart. There are two semilunar valves in the heart: the pulmonary valve, which is located between the right ventricle and the pulmonary artery, and the aortic valve, which is located between the left ventricle and the aorta. These valves prevent the backflow of blood into the ventricles during ventricular relaxation.

Explanation:
If the semilunar valves stop functioning, it will have significant consequences on the overall function of the heart and blood flow. Let's analyze each option to determine which event will not definitely happen if the semilunar valves stop functioning:

A. Atrial contraction:
The contraction of the atria is responsible for filling the ventricles with blood. However, the semilunar valves are not directly involved in atrial contraction. Therefore, even if the semilunar valves stop functioning, atrial contraction can still occur.

B. Ventricular contraction:
Ventricular contraction is the main pumping action of the heart. When the ventricles contract, they push blood out of the heart and into the major blood vessels. The semilunar valves play a crucial role during ventricular contraction by preventing the backflow of blood into the ventricles. If the semilunar valves stop functioning, there will be a significant amount of blood regurgitation back into the ventricles during ventricular relaxation. This will reduce the effectiveness of ventricular contraction and compromise the overall pumping ability of the heart.

C. Flow of blood:
The semilunar valves are essential for maintaining the forward flow of blood. When the ventricles contract, blood is ejected into the pulmonary artery and aorta. The semilunar valves close immediately after the ejection phase to prevent blood from flowing back into the ventricles. If the semilunar valves stop functioning, blood will flow back into the ventricles during ventricular relaxation, leading to inefficient blood circulation.

D. Heart sound production:
Heart sounds are produced by the closing of the heart valves. The closure of the semilunar valves produces the second heart sound (S2) during ventricular relaxation. If the semilunar valves stop functioning, the closure of these valves will not occur, leading to the absence or alteration of the second heart sound.

Conclusion:
From the above analysis, it can be concluded that if the semilunar valves stop functioning, the event that will not definitely happen is the proper closure of the semilunar valves, leading to altered or absent heart sound production. However, it is important to note that the malfunctioning of the semilunar valves will have significant consequences on the overall function of the heart, including reduced effectiveness of ventricular contraction and inefficient blood circulation.

D is answer
bcz when contraction of ventricle complete semilunar valve be close and they make sound dubb.