According to the text, why did Sumer decline? (select all that apply) A. lack of natural defenses made them vulnerable to external threats

B. volcanic eruptions destroyed major cities like Ur and Lagash

C. salinization of the soil led to a dramatic drop in grain production

D. conflict among Sumerian city-states over territory and resources

E. widespread disease led to a decrease in trade with nearby lands

F. Sumerian slaves revolted against the wealthy landowners