I think the correct answers are A irrigation of the crops led to salinization of soil and D Sumer was located on a flat plain that provided no natural barriers to invasion. I think it is A because the sumerians did not know at the time they were destroying their lands with the salt water they provided for their crops. B would not be correct because when I learned about Sumer I didn't learn anything about their lands being on mountains and as for C I did not learn anything on how the land became swampy. I think D would be an answer as well because Sumer is located on a flat plain, and around this time the Sumerians were to caught up on fighting each other to fix the damage that had been done. Tigris and Euphrates rivers gave people in Mesopotamia drinking water and help to irrigate the crops, so it was pretty much their survival weapon, at that time. I hope this helps you, if I am wrong please do correct me with the appropriate answers, I would appreciate the feedback to help me in the future helping anyones questions, thanks and have wonderful day/evening :)