Why did the cost of slave labor double even after the invention of the cotton gin?

a.
the demand for enslaved workers for the cotton fields was high, but the supply of workers was low due to the ban on the slave trade.
b.
northern enslavers refused to sell their enslaved workers.
c.
enslaved workers were in high demand for tobacco and rice farms.
d.
more enslaved workers were being freed at that point so that they could return to africa.