The ornamental design for a refrigerator, as shown and described.
FIG. 1 is a top front perspective view of a refrigerator showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is another side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is another top front perspective view thereof, showing the refrigerator with a removable and transparent upper panel.
The portions of refrigerator shown in the perspective and top views that are shown with internal surface shading represent the same surface depth as the areas surrounded, which are not shaded.
The broken lines shown in FIG. 8 of the drawings shown unclaimed environment only and form no part of the claimed design.