This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Circulation: Heart Valves and Heart Sounds;  Heart Defects, Subject name: Physiology
[{'content': 'S2 is associated with - \nSelect and explain. \nA. Rapid ventricular filling, B. Atrial contraction, C. Closure of semilunar valves, D. Closure of AV valves', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. is \'c\' i.e., Closure of semilunar valves o S, is due to closure of semilunar valves (aortic and pulmonary valves).Feature1st heart sound2nd heart sound3rd heart sound4th heart soundCharacterLow pitched (frequency) slightly prolonged "lub"Shorter high pitched "dup"Soft low pitched weak rumblingArterial heart soundDuration0[?]14 second0[?]11 second0[?]1 second-Frequency25-45 Hz50 Hz-<20HzQCauseSudden closure of mitral & tricuspid valvesClosure of aortic & pulmonary\' valvesRapid ventricular fillingQ d/t inthrusing of blood from atriaVentricular fillingQ d/t atrial contraction causing inthrushing of blood.TimingStart of ventricular systoleJust after end of ventricular systoleBegining of middle third of diastoleImmediately before 1st heart sound (presystolic)HeardS.1 & S2 can be heard with stethoscopeS1 & S4 can not be heard by stethoscope (ear) Q. So oftenly and S5 sometimes can be recorded in phonocardiorgam', 'role': 'assistant'}]