The ornamental design for a ball, as shown and described.
FIG. 1 is a perspective view of a ball showing my new design together with broken lines illustrating disclaimed regions therein;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines are included for the purpose of illustrating shapes that, together with the areas defined thereby and form no part of the claimed design.