Escherichia coli strains (I-IV) with mutations in the lac operon were studied for $\beta$ galactosidase activity, followed by measuring its mRNA and protein levels (Graphs). The arrow indicates the time of addition of a compound (namely IPTG) that induces the lac operon. Based on the results, the correct option(s) is(are)

• A

  In strain-I, the lac operon is not regulated and $\beta$-galactosidase is inactive.
• B

  In strain-II, the lac operon is not regulated and $\beta$-galactosidase is inactive.
• Correct answer
  C

  In strain-IV, the lac operon is not regulated and $\beta$-galactosidase protein is not produced.
• Correct answer
  D

  In strain-III, the lac operon is regulated and $\beta$-galactosidase is active.

Hence, the correct answer is options: 'C', 'D'.