Select all that are true of generalization in extended entity relationship models (EERM). (Check all that are true.) a. Generalization is a top-down process of identifying lower-level, more specific entity subtypes from a higher-level entity supertype. b. Generalization is the bottom-up process of identifying higher-level, more abstract entity supertypes from lower-level entity subtypes. c. Generalization is based on grouping unique characteristics and relationships of the subtypes. d. Generalization is based on grouping common characteristics and relationships of the subtypes