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How did the invention of the cotton gin affect slave labor in the united states? a. there was a forced migration of slaves from the south to the north. b. slave-owning cotton planters were gradually forced to sell their plantations. c. more slaves were purchased to work on cotton plantations in the south. d. the reduced need for labor on cotton plantations led to a decline in slavery. e. slaves were moved from cotton plantations to sugar plantations.