The ornamental design for a sink, as shown and described.
FIG. 1 shows a perspective view of the present design.
FIG. 2 shows a front view of the present design,
FIG. 3 shows a rear view of the present design.
FIG. 4 shows a left side view of the present design
FIG. 5 shows a right side view of the present design.
FIG. 6 shows a top view of the present design; and,
FIG. 7 shows a bottom view of the present design.
The broken lines showing of structural feature is indeed for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.