Gentiobiose, a rare disaccharide found in safron and gentian, is a reducing sugar and forms only D-glucose on hydrolysis with aqueous acid. Reaction of gentiobiose with iodomethane and Ag2O yields an octamethyl derivative, which can be hydrolyzed with aqueous acid to give one equivalent of 2,3,4,6-tetra-O-methyl-D-glucopyran ose and one equivalent of 2,3,4-tri-O-methyl-D-glucopyranose. If gentiobiose contains a beta-glycoside link, what is its structure?