The ornamental design for a bag, as shown and described.
FIG. 1 is a top front perspective view of a bag showing the claimed design;
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 in the figures show portions of the bag that form no part of the claimed design.
The figures are illustrated to show contrasting appearance. Areas of the bag that appear in different shades of gray represent areas with contrasting appearances.