FIG. 1 is a perspective view, showing the new design for an air duct;
FIG. 2 is a left side view thereof, the right side view being a mirror image thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken line showing forms no part of the claimed design, and the shading is provided to illustrate contour only and is not an indication of texture, etc.