The lack of Peace agreements definitely contributed as did the conflicts between tribes, I am not even sure what is meant by the first choice, there was no African nationalism at the time so I eliminate it as the choice also. That leaves C. It makes no sense that traders would use guns as methods of trade. The tribes at the time didn't have guns, slave traders would not want them to be able to kill rather than capture so the only ones who has guns were the traders and they wouldn't have given them up because it gave them the up because it gave them the upper hand over the natives they were trading with.
Hope this helped :)
