C There was a lack of natural barriers that could discourage attack and D, As the fields of Sumer grew saltier, harvests became smaller. The reason I say C and D because A, there was no swamps or any mention about dieased insects in my lesson and for B canals weren't to costly to build. Since the sumerians started to have wars with their own people, Akkadians came in to attack their land since the Sumers people were currently and constantly fighting over who was stronger. And for D, the Sumerians didnt know they were poisioning their own land by giving their crops salt water. So the amount of food went from sky high for many years to down to nothing.