a) The labeling methods for configuration of optically active substance includes D-L labeling and R-S labeling.

b) D-L labeling is to label only with reference to the control standard for configuration of glyceraldehydes, where the configuration of dextro-glyceraldehydes is labeled as D and the configuration of laevo-glyceraldehydes is labeled as L. The compound with the same configuration as D glyceraldehydes is called D and labeled with "D" when named; and the compound with the same configuration as L glyceraldehydes is called L and labeled with "L" when named. D-L labeling method only illustrates the configuration of one chiral carbon atom in the molecule. R-S labeling method is used for the compound containing many chiral carbon atoms. R-S labeling method is to label according to the spatial arrangement of four radicals connected with chiral carbon atom. It illustrates the spatial arrangement of each radical of chiral carbon atom in the compound containing many carbon atoms.

c) As for the lactic acid only with one asymmetric chiral carbon atom in the molecule, its L configuration is S configuration, and D configuration is R configuration.