The ornamental design for a sink, as shown and described.
FIG. 1 is a perspective view of a sink showing my design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a side elevational view thereof; and,
FIG. 5 is a top plan view thereof.
The broken-line disclosure in the views represents portions of the article in which the claimed design is embodied, but which form no part of the claimed design.