The two western african trading centers covered in this chapter were

a.kumbi salem and great zimbabwe

b.great zimbabwe and swahili

c.swahili and jenne-jeno

d.jenne-jeno and kumbi saleh

I beilive it (a Kumbi Salem and Great Zimbabwe

The historian Frederick Jackson Turner argued that the frontier was the key factor in the development of American democracy and institutions; he maintained that the frontier served as a "safety valve" during periods of economic crisis. ☜ (I think that’s the answer)