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

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