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