The correct answer is B.
The invention of agriculture and the beginning of sedentary settlements and civilizations, gave rise to the start of the Neolithic period. Agriculture and sedentarism also brought specialization and division of labor. Specialization allowed to improve the instruments and tools used for each activity (agriclture, animal husbandry, hunting, etc) and therefore productivity was rapidly enhanced. For the first time in history, men produced more than they could consume.
This was the starting point of intentional contact betwen prehistoric clans, because they started to trade their respective production surpluses with each other. Until this moment, each tribe had been been self-sufficient.