Question

The lac operon is shut off when...
a. lactose is present.
b. lactose is absent.
c. glucose is present.
d. glucose is absent.

Answer 1

The lac operon codes for genes involved in lactose metabolism.

So it makes no sense for the cell to expend energy and materials to make proteins that metabolise lactose when there is no lactose inside the cell. In fact, doing this would hurt the cell because it takes up energy and materials the cell needs to do other work...