The ornamental design for a key, as shown and described.
FIG. 1 is a front top left perspective view of a key showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof; and,
FIG. 8 is a front top left perspective view of the portion of the key visible during certain periods of use, where the lighter right portion is not visible when the key is inserted into a keyhole (not shown).