In a bacterial cell, $\beta$ -galactosidase is at high concentrations but $\beta$ -galactoside permease is at very low concentrations.What is the most likely explanation?

A) Both glucose and lactose are at high concentrations.
B) Both glucose and lactose are at low concentrations.
C) The operator has a mutation.
D) The promoter has a mutation.
E) One of the structural genes has a mutation.