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The regulation of metabolic operons, such as the lac operon, and biosynthetic operons such as the trp operon differ in that: A. Metabolic operons are regulated by the substrate of the encoded enzymes; biosynthetic operons, by the product of the encoded enzymes. B. Metabolic operons are regulated by negative regulatory proteins; biosynthetic operons, by positive regulatory proteins. C. Metabolic operons are regulated by cis-acting elements; biosynthetic operons, by trans-acting elements. D. Metabolic operons are regulated by operators; biosynthetic operons, by repressors.