I claim the ornamental design for a key, as shown and described.
FIG. 1 is a perspective view of a key of the new design;
FIG. 2 is a side view of the key of FIG. 1, the other side view being a mirror image;
FIG. 3 is a top view of the key of FIG. 1;
FIG. 4 is a bottom view of the key of FIG. 1;
FIG. 5 is a front view of the key of FIG. 1; and,
FIG. 6 is a rear view of the key of FIG. 1.
The broken lines, which define the bounds of the claimed design, form no part thereof. The broken lines environmental are for illustrative purpose only and form no part of the claimed design.