which factors led to the development of the four river-valley civilizations of mesopotamia, egypt, the indus valley, and china?
choose all answers that are correct.
a. the worship of many nature gods
b. the presence of rivers
c. a surplus of food
d. the building of villages and cities
e. the need to unite for protection from enemies

b and after the building of the canals and the type of floods the rivers had, the crops were abundant, so c is the consequence and there was a surplus of food. Not everybody needed to work in the fields. Some devoted to other jobs like soldiers, rulers, etc. And so they went to live in cities or villages. But in order to distribute the products a strong ruler (it could be called king, priests, army so the state was born) was necessary.     .