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

• A. The lac operon is constitutively active in these cells.
• B.                    They cannot synthesise functional beta- galactosidase.
• C.                    In the presence of glucose, E.coli cells do not utilize lactose.
• D.                    They cannot transport lactose from the medium into the cell.