The ornamental design for a ballast, as shown and described.
FIG. 1 is a perspective view of a ballast, as viewed from below, showing my new design;
FIG. 2 is a perspective view of the ballast shown in FIG. 1, as viewed from above;
FIG. 3 is a left side view of the ballast shown in FIG. 1;
FIG. 4 is a right side view of the ballast shown in FIG. 1;
FIG. 5 is a front elevation view of the ballast shown in FIG. 1;
FIG. 6 is a rear elevation view of the ballast shown in FIG. 1;
FIG. 7 is a top plan view of the ballast shown in FIG. 1; and,
FIG. 8 is a bottom plan view of the ballast shown in FIG. 1.
The broken-line disclosure shown in the views is understood to represent portions of the article in which the claimed design is embodied, but which form no part of the claimed design.