A physical barrier to religious diffusion can be the geography or isolation.
If there's rocky terrain or mountains in the way, believers of a religion will most likely not travel to that place as the trek to it is not safe. Some civilizations can also be isolated from the outside world so many people may not know their location or whereabouts.
slavery in America was often the product of conflict and war between tribes
unlike in America, it was not based on race. Slaves were not able to travel freely in Africa, but they experienced greater freedom within the tribal compound than America slaves. Slaves in Africa were also not necessarily enslaved for life
I must assume your favorite number is 3 since it is the first number and is listed as the first, also there are some decimals if I do it the way I am told. I could try to solve it in another way which would be logical but that would require using both the Sum, Quotient, or Product of an answer AND using the original number (3) again. Sooooooo based on wording... it isn't possible.