The correct answers are A) As the fields of Sumer grew saltier, harvests were less plentiful, B) Wars among the city-states plagued the Sumerians, and C) Sumer was located on a flat plane.
All of the mentioned factors are correct.
Sumeria is considered the oldest civilization on Earth. The Sumerians settled in the fertile territories of the Middle East. More specifically, between the Tigris and Euphrates Rivers. They established important cities such as Ur, Summer, or Uruk. They wrote many Clay Tablets in Cuneiform Acadian writing where Historians can read its famous stories such as the Epic of Gilgamesh and the Enuma Elish, the story of the Creation.