Answer:
C. Was the only kind of labor on that continent
Step-by-step explanation:
Slave has long existed within the African society. Slavery in which most of the slaves are used as forced labor existed in many African kingdoms and societies for hundreds of years.
Slave practices in Africa comes from the major African Kingdoms, especially along the coast, and evidence of widespread slavery practises used as a form of labor can be seen in stateless societies.
Slavery was part of the economic structure of the African countries for many centuries. In the Kingdom of Mali in the mid-14th century for instance, Battuta who visited the Kingdom recounts that the local inhabitants vies with each other through the number of slave servants they had.
With the explanation above, it can be seen that Slave trade was the only labour that existed in the African continent.