I did this problem before, neither answer is right.
No, B is the right answer and it just occured to me why.
I realized just now that this isn't an necessary question but a sufficient question.
It says that Centerville employs more unskilled workers on a full-time basis than all other businesses in town combined. And conclusion like, river stated, mentions RESIDENCE.
so if we were to state "centerville has more unskilled workers among its population than it has skilled workers" the conclusion would follow because we would be limiting the scope to the residence and not to the outside party.
Therefore B is the correct answer. WHEW!