Classify each of the following materials as to whether it is a metal, ceramic, or polymer. Justify each choice: (a) brass; (b) magnesium oxide (MgO); (c) Plexiglas; (d) polychloroprene; (e) boron carbide \left(\mathrm{B}_{4} \mathrm{C}\right)(B 4 C); and f cast iron.
