A major natural grocery chain wants to help address the problem of urb

This is not a well-written argument. A and B are both potential obstacles, but neither is clear enough to be a definitive weakener. Answer choice A depends on an assumption that the stores won't offer quick, easy food, and/or that people won't be encouraged to try something other than their normal preference. So I can see why we wouldn't call it a weakener. However, B relies on the assumption that people wouldn't be willing to pay a bit more for healthy options. Yes, they're in impoverished areas, but that doesn't mean that the presence of healthy options that cost a bit more wouldn't be encouraging. This would be a more useful point if the argument were making bold claims about the sales or profitability of this store. But since the conclusion is just about "encouraging," it's hard to call B much of a weakener.

By the way, one clue that this is an amateur question is the present tense: "A chain wants . . . it decides . . . " That's the tense of jokes ("A priest and a rabbi walk into a bar . . . "), not GMAT arguments. The GMAT would use present perfect: "A chain HAS decided," etc.