Escherichia coli bacteria can control gene expression during changes in environmental conditions. Analyze the example of gene expression of the bacteria below: Which statement best explains how the genes are being regulated?

a. The genes are off when the initiating chemical is present.
b. The initiating chemical bonds directly to the DNA and blocks the metabolism.
c. The operon is turned on only when the stimulating chemical is present.
d. The genes are turned off by the unbound repressors.