I would say inside due to less friction.
All of them were river valley based civilization remains, thought we have many cities we have many cities in uneven terrains and arid regions , which may either have been outposts for trade, or were well used with water harvesting techniques to use the little available water through the rains . Created artifacts indentify as “art”, religion and priestly class that maintained rituals and douctrines
Explanation:
Fertile productive soils are soils for which internal and external factors relating to the soil itself, such as heat and rain, are suitable.
C. Ecuador
it is located in the western area of south america