Jerome is learning how the model of the atom has changed over time as new evidence was gathered. He has images of four models of the atom, but they are not in the correct order. An image at left labeled W with overlapping red and blue balls in the center with a circular fuzzy green cloud outside them. An image at center left labeled X with a purple ball in the center surrounded by overlapping concentric black ovals, each with a small green ball on each of the 6 ovals. An image at center right labeled Y with a large black cross in a purple circle with a black line around the purple, with 10 small green balls dispersed within the purple circle. An image at right labeled Z with a purple center outlined in black with two concentric black circles around the center, the inner circle having 2 small green balls on it and the outer circle having 8 small green balls on it. In what order should Jerome put these models to show the development from the earliest model of the atom to the most recent one?