Which item describes an accurate cause-and-effect relationship that existed after the fall of the roman empire

A. Decline in trade → shift to a rural society

B. Germanic tribes invading Roman territories → strengthening of central government

C. Increasing urban populations → Germanic tribes invading Roman territories  

D. Shift to a rural society → strengthening of central government

Explanation:

its A. just took test

it broke up the ottoman empire (just a small fact) and the turkish leaders lost many soldiers.