FIG. 1 is a perspective view of a sink showing our new design; FIG. 2 is a top plan view thereof; FIG. 3 is a bottom view thereof; FIG. 4 is a front elevational view thereof; FIG. 5 is a rear view thereof; FIG. 6 is a right side view thereof; and, FIG. 7 is a left view thereof. The broken lines showing portions of the claimed design form no part thereof.