Zachary Is Looking at a Reversible Figure That First Appears

Zachary is looking at a reversible figure that first appears to be a vase and then appears to be two faces.His perception of the figure keeps switching between these two interpretations.What causes the switch in perception

A)  The Gestalt principle of simplicity doesn't work for reversible figures. 
B)  The figure-ground distinction in reversible figures is often ambiguous. 
C)  The Gestalt principles of proximity and closure are both at work in reversible figures. 
D)  Reversible figures cause people to experience the phi phenomenon.