The graph below shows the pressure changes in the left atrium, left ventricle, and aorta of a healthy mammal during a single cardiac cycle. Points WWW, XXX, YYY, and Z Z\,Z represent points where the pressure curves intersect.
Which of the following statements correctly describes the status or function of the valves at the labeled points?
At point WWW, the atrioventricular valve opens to allow blood to enter the left ventricle, initiating ventricular systole.
At point XXX, the semilunar valve in the aorta opens, allowing blood to flow from the left ventricle into the aorta.
At point YYY, the atrioventricular valve closes to prevent blood from flowing back into the left atrium.
At point ZZZ, the semilunar valve closes because aortic pressure has dropped below atrial pressure.
