Why Was Sumer an Uninviting Environment for the First Cities

Why was Sumer an uninviting environment for the first cities?

A) The area has no natural defenses.
B) The soil was sandy and the rivers flooded unpredictably.
C) The marshy land between the rivers was fertile breeding ground for malaria and other deadly diseases.
D) The rivers were largely unnavigable during long periods of the year,making trade difficult.
E) The region had no forests for timber or usable stone to quarry for building materials.