When glucose is methylated (with dimethylsulfate in aqueous base) and then hydrolyzed (with dilute acid), the only product is 2,3,4,6-tetra-O-methyl-D-glucose. But when fructose is methylated and then hydrolyzed, it forms a mixture of isomers -- 33% 1,3,4,6-tetra-O-methyl-Dfructose, and 67% 1,3,4,5-tetra-O-methyl-D-fructose. What does this experiment indicate about the structure of D-fructose in aqueous solution?