Answer:
Correct answer is B. settle next to a river.
Explanation:
B is correct answer as people had to move near the rivers, so they could provide their basic needs for life. Of course with this techniques they didn't need to move that much.
A is not correct because agriculture was their main preoccupation.
C is not correct because this was the main way the moved water from one place to another.
D is not correct as Southern Mesopotamia was the most fertile part of this area.
