i think its A, if the factories caused pollution, then people would want to move away from the pollution, causing the population to decrease. if the factories benefited the city and made it stand out to others, then it would be B. If the factories didnt make a difference to the city, it would be C.
personally, i think its A, but if there is a story/passage with it, then read it and see for anything i said. sorry if i didnt help.
I'd go with A. hope it helps!:)