The ornamental design for a sink, as shown and described.
FIG. 1 is a perspective view of the sink.
FIG. 2 is a front view of the sink.
FIG. 3 is a rear view of the sink.
FIG. 4 is a top view of the sink.
FIG. 5 is a bottom view of the sink.
FIG. 6 is a left exterior side view of the sink.
FIG. 7 is a right exterior side view of the sink.
FIG. 8 is a sectional view of the sink from the front, as indicated in FIG. 4; and,
FIG. 9 is a sectional view of the sink from the left, as indicated in FIG. 5 (inverted).
The arrows and corresponding lines in FIGS. 4 and 5 indicate the sectional views for FIGS. 8 and 9 respectively (with FIG. 9 inverted to be right-side-up).