I do not know the answer, but if I were to guess, that it would be the one about plantations.
Plantations were hotspots of manual labor, specifically slaves. And when slaves were outlawed the plantation owners lose there main source of “profit” (aka the need to not pay workers for manual labor and instead use that money to get more laborers) it seems like the logical answer to me.
Also using process of elimination we know that the war could have spilt the country in two. So the union was indeed saved.
