The ornamental design for a bed, as shown and described.
FIG. 1 is a front elevational view of a bed showing my new design, the bottom being plain and unornamented and the mattress shown in broken lines to indicate that it is not part of the design claimed;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is an elevational rear view of the bed; and,
FIG. 4 is a front perspective view of the bed with the mattress shown as in FIG. 1.