Explain why (a) inactivation of the O2 or O3 sequence of the lac operon causes only a twofold loss in repression, and (b) inactivation of both O2 and O3 reduces repression 70-fold. The following passage wasn't included with the question, but it might be helpful: "The lac operon actually contains three operator sequences to which lac repressor binds with high affinity, known as O1, O2, and O3. O1, the primary repressor-binding site, was identified through its protection by lac repressor from nuclease digestion. The 26-bp protected sequence lies within a nearly twofold symmetrical sequence of 35 bp. O1 overlaps the transcription start site of the lacZ gene. The operator sequence O2 is centered 401 bp downstream, fully within the lacZ gene, and O3 is centered 93 bp upstream.