The ornamental design for a bed, as shown and described.
FIG. 1 is a front perspective view of a bed;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a back elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left elevation view thereof; and,
FIG. 7 is a right elevation view thereof.
The bed is shown with a symbolic break in its length. The appearance of any portion of the bed between the break lines forms no part of the claimed design.