What led to the start of early civilizations

Human beings were present in this earth for a long time but they settled down to create civilization after a long time. Initially the humans hunted and moved from place to place for need of food. Slowly it became hard for humans to hunt and find food. The humans started growing crops and settling down beside river banks for pure and clean water. The urge for growing crops in the fertile river banks led to the humans creating houses for living. This was the start of civilization and settlement.   

Humans were once nomadic people. They would move from place to place in search of animals to hunt. But then they learned agriculture and settled down to farm. Because of this change, they started to create civilizations. More and more people settled down near each other.

I hope this helped :)

Ithink the answer is a.