The operon theory of gene regulation explains how the production of proteins is regulated in bacteria. A complex of genes, the operon consists of two regions: a coding region, for enzymes involved in a metabolic pathway, and a regulatory region that determines whether the gene is switched "on" or "off." There are two types of operons. In an inducible operon, protein production is "off" by default, and turns "on" when a sufficient quantity of inducer is present. In a repressible operon, protein production is "on" by default, and turns "off" when an active repressor complex is present.

The lac operon is an example of an inducible operon. Normally, the lac operon's protein production is inactive; since the lac repressor is bound to the regulatory region, the gene does not produce enzymes. However, when lactose, or milk sugar, is present in high concentrations, the lac repressor binds instead to the lactose. Since the regulatory region is now free of a repressor, the gene activates and begins to produce proteins.

The process is neatly self-regulating. The proteins produced by the now-activated genes lactose-digesting enzymes, which in turn down the lactose. Once all lactose is broken down, there is no remaining lactose that can bind to the lac repressor. The repressor returns to the regulatory region, turning the operon "off" and arresting the production of enzymes and the digestion of lactose.

Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible.
B. Lactose is the inducer of the lac operon.
C. The lac repressor prefers to bind to lactose rather than the regulatory region.
D. The repressible operon does not normally have anything bound to its regulatory region.
E. Lactose is the main source of energy for bacteria.

Source is Kaplan CAT. Please choose your answer choice and explain why it is correct by quoting some sentences from the passage.