The ornamental design for a refrigerator, as shown and described.
1. Refrigerator
1.1 is a perspective view of a refrigerator showing my new design;
1.2 is a front elevation view thereof;
1.3 is a rear elevation view thereof;
1.4 is a left-side elevation view thereof;
1.5 is a right-side elevation view thereof;
1.6 is a top plan view thereof;
1.7 is a bottom plan view thereof;
1.8 is another perspective view thereof, wherein the inside of the right outer door is seen through a ‘see-through’ type of window.
1.9 is another perspective view thereof, wherein the right outer door is in an opened position; and
1.10 is another perspective view thereof, wherein each of the right inner and outer doors is in an opened position. The broken lines shown are included for the purpose of illustrating unclaimed portions of the refrigerator and form no part of the claimed design.