During the Cardiac Cycle, Why Do the Aortic and Pulmonic

During the cardiac cycle, why do the aortic and pulmonic valves close after the ventricles relax?

A) Papillary muscles relax, which allows the valves to close.
B) Chordae tendineae contract, which pulls the valves closed.
C) Reduced pressure in the ventricles creates a negative pressure, which pulls the valves closed.
D) Blood fills the cusps of the valves and causes the edges to merge, closing the valves.