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

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