We claim the ornamental design for a hearing aid, as shown and described.
FIG. 1 is a front perspective view of a hearing aid of the present invention;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a first side view thereof;
FIG. 4 is a second side view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is a cross-section view taken along the line 9-9 of FIG. 3 with the internal parts omitted.
The long-short dash lines represent the bounds of the claim, and all other broken lines are included for the purpose of illustrating portions of the hearing aid that form no part of the claimed design. None of the broken lines form a part of the claimed design.