The ornamental design for a bag, as shown and described.
FIG. 1 is a perspective view of an embodiment of a bag;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a side elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a perspective sectional view taken along line 6-6 of FIG. 4 showing the interior of the bag.
The broken lines shown are included for the purpose of illustrating the unclaimed portions of the article and the article's environment and form no part of the claim.