The ornamental design for a sink, as shown and described.
FIG. 1 is a rear, top perspective view of a sink showing our new design;
FIG. 2 is a front view of the sink;
FIG. 3 is a rear view of the sink;
FIG. 4 is a left side view of the sink;
FIG. 5 is a right side view of the sink;
FIG. 6 is a top view of the sink;
FIG. 7 is a bottom view of the sink; and,
FIG. 8 is a front perspective view of the sink.
The broken lines immediately adjacent the shaded areas represent the bounds of the claim, 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.