The ornamental design for the sink, as shown and described.
FIG. 1 is a perspective view of the sink showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is another perspective view thereof.
The broken lines illustrate unclaimed features that form no part of the claimed design. The bottom view is not a matter of ornamental concern and has been omitted from the drawings.