E Coli Regulates the Biosynthesis of the Amino Acid Tryptophan

E. coli regulates the biosynthesis of the amino acid tryptophan through the tryptophan (trp) operon. When tryptophan levels are high inside the cell, tryptophan binds to a regulatory protein, which then binds to the operator site of the operon. This shuts down transcription of the genes encoding enzymes needed for tryptophan biosynthesis, and tryptophan biosynthesis is halted. The trp operon is an example of

A) positive control because binding of a repressor blocks transcription.
B) negative control because binding of a repressor blocks transcription.
C) positive control because binding of an activator stimulates transcription.
D) negative control because binding of an activator stimulates transcription.