Why did the institution of slavery develop in africa?

a. africa had too many people.
b. slaves were valued as goods to be sold and traded.
c. european traders introduced slavery to west africa.
d. africa had an excess of land and a shortage of labor.

i'm not sure if it's b or c. !

hi would love to but need a bit more info

we pointed them north: recollections of a cowpuncher by e.c. “teddy blue” abbott tells us of the history of nomad cowboys in north america. as the   frontiers began moving west, trails of cowboys of the 1870s and ’80s drove herds of cattle from texas to the northern ranges and railheads, and then rode back south to do it again. they gradually pushed   native americans out of their lands, until in 1889 the   government transferred white   colonists the indian land.