The correct answer is B. The invention of techniques for planting crops and herding animals.
Explanation
In ancient times, human communities used to be nomads, that is, they constantly moved from one place to another in search of food, animals for hunting, caverns or places to take refuge and nearby water sources; however, with the discovery of the agriculture, they had constant food sources, in addition, grazing was an activity that allowed them to have meat and milk for food. Despite this, crops and animals need water to survive, for this reason, they had to settle close to the rivers becoming river valley communities. So, the correct answer is B. The invention of techniques for planting crops and herding animals.
