After the Civil War the slaves were freed, but they were not yet full-fledged citizens in reality. It started with the Reconstruction, which involved a number of actions and laws that were in the function of helping former slaves. Despite this, southern planters in all possible ways obstructed the freedom of former slaves, and one of them was the Black Code. It was part of a new southern government that tried to put slaves in a position closer to slavery, and forced them to work on plantations without civil rights. A part of the Republican Party tried to help former slaves, so they also risked their lives because of the Black Code. Skilled work was always better paid than other basic physical jobs, so this job was not reserved for former slaves.
The answer is: D.
