The answer is A.
A: If last year new retail stores opened offering high quality merch at competitive prices, then maybe the sales volume didn't really decrease. After all, the Post-Gazette only surveyed the retails stores
in business both of those years, so the new stores would likely get excluded. If they were included in the survey, we could expect to see the sales volume either the same or increased (since stores offer quality merch at competitive prices). In that case, the conclusion wouldn't really make sense anymore-- Chabberton did not actually have a smaller sales volume.
B: This strengthens. If residents are shopping at stores other than Chabberton, then it makes sense why sales volume is lower. However, this choice is set up without describing a specific time period. If this is a general trend, it doesn't offer any explanation as to why last year has lower retail sales than the year before.
C: Not really sure what C implies. If they advertise regularly, should the sales volume be higher? Maybe, maybe not. There's no clear connection. Additionally, this describes a general trend, whereas the conclusion is comparing the retail sales over the last two years.
D: Volume of business is not equal to sales volume, so greater volume of business needn't have anything to do with the sales volume.
E: Pseudo-weakener. We might be tempted to think that the sales weren't actually so small last year, but the conclusion notes last year's sales
in relation to the year before last. So it doesn't matter that the year before last had record sales.