The ornamental design for a sink, as shown and described.
FIG. 1 is a front top perspective view of a sink showing our new design;
FIG. 2 is a rear top perspective view thereof;
FIG. 3 is a front bottom perspective view thereof;
FIG. 4 is a rear bottom perspective view thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a right-side view thereof;
FIG. 8 is a left-side view thereof;
FIG. 9 is a top plan view thereof; and,
FIG. 10 is a bottom plan view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.