The ornamental design for a bed, as shown and described.
1. Bed
Fig. 1.1 is a perspective view of the design; fig. 1.2 is a left side view thereof, the right side being a mirror image; fig. 1.3 is a top view thereof; fig. 1.4 is a front view thereof; fig. 1.5 is a rear view thereof; fig. 1.6 is a perspective view of the design in alternative position; fig. 1.7 is a side view of the design in alternative position; fig. 1.8 is a top view of the design in alternative position; The broken lines shown on the bed is for the purpose of illustrating portions of the article that form no part of the claimed design.