How did rivers affect the interaction between prehistoric groups?

a. they protected groups from invasion.
b. they invited interaction with other regions.
c. they kept groups separate most of the time.
d. they let groups divide and form unique cultures.

assuming that you are talking about the one in egypt: hecka oil

explanation:

after the aryan's settled in india they changed for the better i bellieve they changed from nomads having to keep moving to being farmers   they were aswell avaible to turn the ganges river into farmland.   which is great they added a big affect on farming as we know of it today.