Option (3) is the correct answer because during joint diastole, blood moves freely from atrium to ventricle as atrioventricular valve remain open during joint diastole. Option (4) is incorrect because decrease in ventricular pressure, during ventricular diastole closes semilunar valves to produce ‘dub’ heart sound. Option (1) is incorrect because SA node generates action potential to stimulate atrial contraction. Option (2) is incorrect because bicuspid and tricuspid valves open due to pressure exerted by blood present in atria and decrease in pressure in ventricles during ventricular diastole.