The ornamental design for a bag, as shown and described.
FIG. 1 is a perspective view of the bag of the present invention;
FIG. 2 is a front elevation view of the bag of the present invention;
FIG. 3 is a rear elevation view of the bag of the present invention;
FIG. 4 is a top plan view of the bag of the present invention;
FIG. 5 is a bottom plan view of the bag of the present invention;
FIG. 6 is a first side view of the bag of the present invention; and,
FIG. 7 is a second side view of the bag of the present invention in use.
The broken lines showing portions of the bag are included for the purpose of illustrating environmental structure and form no part of the claimed design.