In the $lac$ operon,which of the following acts as an inducer?

At which level is gene expression regulated in the $lac$ operon of $E. coli$?

$A$: Operator gene is functional when it is not blocked by repressor.
$R$: Regulator gene produces active protein only which acts on operon system in $E. coli$.

Select the correct option for $\beta$-galactosidase.

Consider the following statements of gene regulation and select the correct statements $:$
$(A)$ Expression of genes is regulated by metabolic,physiological,or environmental conditions.
$(B)$ Galactose can act as an inducer for the lac operon.
$(C)$ The operator region is adjacent to the promoter elements in most operons.
$(D)$ In the lac operon,lactose is transported into the cell through the action of permease.
$(E)$ Regulation of the lac operon by a repressor is referred to as positive regulation.

In $lac$ operon,the switching on or switching off of the operator is achieved by . . . . . . .