Why did the ancient sumerians look for new ways to produce their food?

Sumer is the name of the earliest known civilization in Mesopotamia, and one of the oldest in world history.  

Their growth can be attributed mostly to their success at agriculture, which enabled them to settle in large groups. There were several factors that contributed to the development of new technology and mechanisms of food production.

The first factor is the fact that Sumerians were settled in the valleys of the rivers Tigris and Euphrates, however, the area was mostly dry in terms of precipitation. This meant that farmers had to rely on the rivers, and not on rain, for irrigation. This lead them to develop a complex systems of canals that irrigated the crops.  

Their irrigation system was so successful that Sumerian population rapidly expanded. Moreover, the soil was fertile because of the continuous flooding of the rivers. This flooding, however, meant that canals had to be constantly tended to and rebuilt, and Sumerians used a large labour force for this purpose.

Pressure to provide enough food for a growing population led them to continue to innovate, and this self-reinforcing cycle led to the development of technologies such as ploughs that allowed them to cultivate large areas of land with ease.

four sitting presidents have been assassinated while in office but the first was abraham lincoln,

  the common sense as a pamphlet written by thomas paine. it promoted the independence of the colonies from british rule. the distribution of this pamphlet was great, it reached people everywhere and it was even read aloud at places people met. it also unite the colonists before the revolutionary war.

answer is d