A researcher is studying the regulation of the lac operon in E. coli and has created the following strains with different genotypes:

Strain 1: I⁺ P⁺ O⁺ Z⁺ Y⁺
Strain 2: I⁺ P⁻ O⁺ Z⁺ Y⁺
Strain 3: I⁺ P⁺ O⁺ Z⁻ Y⁺
Strain 4: I⁺ P⁺ O⁺ Z⁺ Y⁻

The researcher wants to determine whether -galactosidase is produced in the presence and absence of the inducer for each genotype. The results of the experiment are shown in the table below:

 Strain   Inducer present     Inducer absent 

     1                    +                        - 

     2                   -                         - 

     3                   +                        + 

     4                   -                         -
 Which of the strain(s) is/are lac⁺?