31. In the trp operon of E. coli, what is the specific role of tryptophan in the negative regulation of gene expression? (A) It acts as an inducer that inactivates the repressor. (B) It binds to the operator to block RNA polymerase directly. (C) It functions as a corepressor that activates the trp repressor. (D) It increases the level of cyclic AMP to stimulate the promoter.