The point of the question was to evaluate whether opening costmart can harm economy and not if costmart opens, will the bankruptcy rate go up. The bankruptcy going up, even if it hurts the economy to some extent, could be offset by some other benefit of opening costmart
Option C caters to the latter. It gives an answer to some degree that if most residents shop within city, then there will be many stores in the city and if costmart comes, all those stores will be hit and may go bankrupt. On the other hand, if most of them shop outside the city, then the city's purchases are happening from outside and the costmart opening will not impact THOSE STORES.
In the second scenario, there can still be stores operating in the city which are not serving the local population. Even if they are impacted, the local economy will still suffer. Hence B is not the correct answer
C asks the question if there are economic benefits as well. Depending on the answer, the benefots can be weighed against the economic loss and a decision to allow costmart to open can be arrived at.
This is my reasoning for choosing C over B. I chose B by the way in my try. If
Sajjad1994 or
GMATNinja can take a look