Which of the following statements about slavery in the early 19th century is true?

a.) The federal government abolished the international slave trade in 1808, but slavery proliferated nonetheless.

b.) Gradual emancipation was instituted in 1808 in the South to help its economy transition away from slavery.

c.) Slavery was illegal starting in 1808, but southern states nullified federal law.

d.) After 1808, slaves could no longer buy their freedom, and free southern blacks were forced into slavery.