In What Way Did the European Slave Trade Enable Centralizing

In what way did the European slave trade enable centralizing states to expand more rapidly?

A) Slaves were traded for firearms that allowed expanding states to overpower their neighbors, resulting in more slaves.
B) The Europeans rapidly created military alliances and added their armies to those of their slave-trading allies.
C) The European slave trade weakened the states of central and western Africa, allowing the centralizing states of eastern Africa to expand without competition.
D) The slave trade was restricted to the coasts, leaving the political units of interior Africa free of European interference.