Describe the characteristics of the first heart sounds and its intensity at the apex of the heart and at the base.
a) S1 is low-pitched and longer, heard best at the base of the heart.
b) S1 is high-pitched and shorter, heard best at the apex of the heart.
c) S1 is caused by the closure of the semilunar valves, heard best at the apex of the heart.
d) S1 is caused by the closure of the atrioventricular valves, heard best at the base of the heart.
