A recombinant protein is to be expressed under the control of the lac promoter and operator in a strain $\textit{E.coli}$ having the genotype lacI$^+$ crp$^+$ . Even in the absence of inducer IPTG, low levels of expression of the recombinant protein are seen (leaky expression). Which one of the following should be done to minimize such leaky expression?
- Addition of lactose to the medium
- Removal of all glucose from the medium
- Addition of excess glucose to the medium
- Addition of allo-lactose to the medium