'Lub' sound of Heart is caused by the ______

The correct answer is option (4) because LUB is the first heart sound and it is produced by closing of AV valves; (tricuspid valve in between right atrium and right ventricle) and (bicuspid /mitral valve in between left atrium and left ventricle), during ventricular systole. - Option (1) is incorrect as closure of the semilunar valves produce the second heart sound called 'DUB'. - Option (2) is incorrect because opening of valves are not responsible for production of heart sounds 'LUB' and 'DUB'. - Option (3) is incorrect because opening of semilunar valves does not produce only heart sounds.