Which of the following switches off the $lac$ operon?

Which of the following is not a structural gene of the $Lac$ operon?

$E. coli$ cells with a mutated $Z$ gene of the $lac$ operon cannot grow in a medium containing only lactose as the source of energy because:

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

In the Operon concept,the regulator gene functions as:

The lactose present in the growth medium of bacteria is transported to the cell by the action of