Answer is D.
Heart sounds are made by valves closing. As the ventricles contract during systole, the pressure in them eventually exceeds that in the atria. This causes the atrioventricular caves to close (they remain open only when atrial pressure exceeds ventricular pressure). This is the first heart sound.
The second heart sound occurs when the semilunar valves close at the beginning of diastole. Again it’s a question of pressure gradients. When the ventricles stop contracting, the pressure in the tube they connect to (pulmonary trunk or aorta) eventually exceeds the ventricular pressure and caused the semilunar valves to close.
