What is the paradox as related to slavery in the United States?

Question 1 options:

Slaves predominantly returned to Africa

That slavery was still allowed in a free nation

That slavery was outlawed in only part of the United States

There was no slavery in the United States

I want to say it is A.) That slavery was still allowed in a free nation or D.) There was no slavery in the United States

Explanation:

But i don't know just tell me if i got that wrong which i hope i got it right

crop rotation kepy different plants in the soil instead of once constintly. when switching out plants you have different nutrients being absorbed at different speeds and different roots growing differently therfore the spoil and water flow have to adapt and carry fresh nutrients to carry the constant changes that were going on in the soil.