Which lettered area best marks the area within the dna sequence where rna polymerase would bind to ensure transcription of pmoa, pmob and pmoc as a unit?

I found the exercise on the internet and attached is the representation of the operon pmo.

The lettered area that best marks the DNA sequence where RNA polymerase would bind is a.
To ensure that all genes were transcripted, the RNA polymerase would have to start opening the double-stranded DNA before the first gene, considering the direction towards the 5' region of the sense strand. For this to happen we may consider areas a and b, as area c is already part of the gene transcription, and, therefore, the RNA polymerase would not bind (it has its own binding site), and all the other areas will not include the transcription of the pmoC gene. Between areas a and b, we should consider the different parts that constitute an operon. An operon has a binding site for the RNA polymerase, which is usually the first part of the operon. After the RNA polymerase binding site, and before the actual gene of interest, there is also an operator, which regulates the actual gene expression by repression. So, a is to be considered the RNA binding site and b the operator. 

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