The ornamental designs for a sink, as shown and described in the accompanying drawings.
FIG. 1 is a top view of a design of a sink;
FIG. 2 is a bottom view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a perspective view thereof.
The broken line showing of the sink is for the purpose of illustrating environmental structure and forms no part of the claimed design. The broken line(s) which define the bounds of the claimed design form no part thereof.