This brought an end to the great west african empires.

a. an increase in countries producing their own goods and not needing to trade

b. the arrival of christianity and the decline of islam

c. enemy nations using sophisticated warfare to overpower the outdated tactics of the empire

d. emperors being unable to further the development of technology and learning

Out of the choices given, enemy nations using sophisticated warfare to overpower the outdated tactics of the empire brought an end to the great West African empire. The correct answer is C. 

It depends on the surface of the levels. the rougher the surface, the greater is the frictional force. for rougher surfaces, you would need to exert greater force to move the object. the concept is that, you must overcome the frictional force in order to bring the object into motion. this is the reason why we add lubricants to the joints so they could easily move. this answer was on