The ornamental design for a sink, as shown and described.
FIG. 1 is a perspective view of a sink;
FIG. 2 is a top plan view of the sink of FIG. 1;
FIG. 3 is a bottom plan view of the sink of FIG. 1;
FIG. 4 is a front view of the sink of FIG. 1;
FIG. 5 is a rear view of the sink of FIG. 1;
FIG. 6 is a right side view of the sink of FIG. 1; and,
FIG. 7 is a left side view of the sink of FIG. 1.
The broken lines showing the rear, side and bottom of the sink are included for the purpose of illustrating a portion of the sink that forms no part of the claimed design.