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⁺?

• A. Strains 1 and 2
• B. Strains 3 and 4
• C. Strains 2 and 3
• D. Strains 1 and 3

Answer: (D)