Following are a few statements made regarding the lac operon.

A. The LacZ, Lacy and LacA are encoded by a single transcript.

B. The three proteins are translated as a single precursor and then processed.

C. In the presence of glucose, lactose can upregulate the operon.

D. Isopropyl thio β-D-galactoside (IPTG) is a gratuitous inducer.

Which one of the following options represents the combination of all correct statements?