GMAT Sentence Correction (SC) | Re: In the textbook publishing business, the second quarter is

I remember SC manhattan said that when "and" is used to link two clauses, there is always a comma right before "and". Thus, in order for (A) to be correct, it should look like this: "low, and marketing expenses are high as companies prepare".
I know that questioning an OA is stupid, but can someone please enlighten me?

---