The ornamental design for a chain, as shown and described.
FIG. 1 is a front view of a first embodiment of a chain, shown in a closed position with the ends connected;
FIG. 2 is a front view of a second embodiment of a chain, shown in a closed position with the ends connected;
FIG. 3 is a front view of a section of the chain in FIGS. 1 and 2, shown in an open position with the ends apart;
FIG. 4 is a side view of the section of the chain shown in FIG. 3;
FIG. 5 is a back view of the section of the chain shown in FIG. 3; and,
FIG. 6 is a side view of the chain section from the opposite side of FIG. 4.
The broken lines in the drawings, which illustrate the clasp features and the small circular jump rings between the links, depict portions of the chain that form no part of the claimed design.