The correct answer should be either a or b.
A - The amount of slaves went down because the cotton gin replaced many people.
B - When something is mass-produced, you can make it for less and earn more money
C - This option just doesn't make sense considering the fact that cotton gins were a positive invention. If they weren't positive they wouldn't have kept them.
D - This could be true but it is very unlikely considering the context of the question. As I stated above, the cotton gin replaced many slaves (everywhere, not just in the South).
I am more leaning toward A because that's what I learned; but, B could also work.
