The ornamental design for a knife, as shown and described.
FIG. 1 is a left side elevational view of the design of the present invention for a knife, the right, opposite, side of which is a mirror image thereof;
FIG. 2 is a top plan view of the knife of FIG. 1, the bottom plan view of which is a mirror image thereof;
FIG. 3 is a rear elevational view of the knife of FIG. 1; and,
FIG. 4 is a front elevational view of the knife of FIG. 1.