How Did the Ecological Conditions of Mesopotamia Contribute to a Dramatic

How did the ecological conditions of Mesopotamia contribute to a dramatic rise in population density beginning around 4000 B.C.E.?

A) A series of volcanic eruptions forced villagers on the outskirts to seek safety in larger and more central settlements.
B) An increase in annual rainfall ensured more reliable crop yields.
C) A decrease in rainfall prompted farmers to innovate new ways of storing food and prevented villagers from simply migrating to find new sources of food.
D) An increase in annual rainfall caused massive flooding and drove villagers from their homes toward urban centers.