UKMT Pink Kangaroo, 2015

A large set of weighing scales has two identical sets of scales placed on it, one on each pan. Four weights $W, X, Y, Z$ are placed on the weighing scales as shown in the left diagram. Then two of these weights are swapped, and the pans now appear as shown in the diagram on the right. Which two weights were swapped?

(A) $W$ and $Z$ (B) $W$ and $Y$ (C) $W$ and $X$ (D) $X$ and $Z$ (E) $X$ and $Y$