The ornamental design for a sink, as shown and described.
FIG. 1 is a perspective view of a sink of the new design;
FIG. 2 is a front elevational view of the sink of FIG. 1, the rear elevational view being a mirror image;
FIG. 3 is a right side elevational view of the sink of FIG. 1, the left side elevational view being a mirror image;
FIG. 4 is a top plan view of the sink of FIG. 1; and,
FIG. 5 is a bottom plan view of the sink of FIG. 1.
The portions of the Figures shown in broken lines are not part of the claimed design and are for illustration only. The pairs of wavy lines in the Figures represent boundaries of the claimed design and are not part of the claimed design. The portions between such pairs of wavy lines are of indeterminate length.