The ornamental design for a refrigerator, as shown and described.
FIG. 1 is a front perspective view of the refrigerator showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a partial enlarged view of a portion of FIG.1; and,
FIG. 9 is a perspective view showing the design in a state of use.
The broken lines depict portions of the refrigerator that form no part of the claimed design. The long dash-short dash-long dash broken lines on FIG. 1 and FIG. 8 depict the boundary of the enlarged view on FIGS. 8, taken from FIG. 1, and form no part of the claimed design.