Answer:
B. It was hard to find enough food in one area.
Explanation:
During our hunter/gathering days, humans haven't possess the knowledge that is necessary to obtain our own foods. So, humans constantly seeking for the animals/fruits that exist around them.
These resources are limited. If there are too many humans that live in one same area, they wouldn't be able to find enough resources to accommodate their basic needs. When this happen, humans often engaged in conflict that made them hurt one another. So, humans population during that time period decided to live in small groups instead in order to avoid all these.