Hello there! The cotton gin would be your answer :)
During the pre-civil war era, cotton was extremely hard for slaves to do by hand, and slowed down production of cotton. This machine, created by Eli Whitney and helped speed the process of collecting cotton which dramatically increased the profit of Southern plantations.
I hope this helped! If you have any more questions just comment or message me :) I can give a more in-depth lesson about the effects of the cotton gin if you'd wish. Good luck with school!
Here's a picture showing how the cotton gin works (found on slideshare.net)
A slave is a person that belongs to another one and is treated as property. Serfs, I'm not so sure about this now, were renting the land out from the landowners.
The answer is assembly line.
