**PLEASE HELP!!**

There is considerable evidence that irrigation may have played a pivotal role in the foundation of the earliest civilizations, such as that of Sumer in the Tigris-Euphrates valley. The reasons for the influence of irrigation are twofold. The development of irrigation allowed for extremely efficient agricultural production, creating the surplus of food resources that must serve as the foundation for any civilization. Furthermore, constructing the elaborate system of canals and drainage networks was a task of tremendous complexity. The centers of commerce, administration, and science that accomplished the task eventually blossomed into the cities that served as the cornerstone of Sumerian civilization.

**The second sentence ("The reasons…are twofold") refers to:**


the construction of elaborate canals and drainage systems
a need for a food surplus and a successful economy
the establishment of successful agriculture and the foundation of cities
the development of commerce and administration
the complexity and success of the irrigation systems