WordNet sense 1 (an odorless very poisonous gas that is a product of incomplete combustion of carbon):

carbon monoxide, carbon monoxide gas

WordNet sense 2 (a hard ferromagnetic silver-white bivalent or trivalent metallic element; a trace element in plant and animal nutrition):

cobalt

WordNet sense 3 (one who refuses to serve in the armed forces on grounds of conscience):

conscientious objector

WordNet sense 4 (a state in west central United States in the Rocky Mountains):

Colorado, Centennial State