Which contributing factors political, economic, military were the most significant in the fall of the western roman empire?

During the 1960s, the government of modern egypt built the aswan high dam to store water for drinking and irrigation, control flooding, and generate hydroelectric power.

Who ever answers first gets ! trade in the vedic age led to a.the rise of kingdoms because of the importance of controlling land and long-distance travel routes b.the building of an extensive system of roads because of the difficult terrain c.the creation of a large banking class because of the need to exchange different currencies d.the development of a centralized government that controlled the economic decisions of its people

Which event is considered to be start of the "hellenistic era"? a) the founding of the roman empire b) the death of alexander the great c) the end of the peloponnesian wars d) the founding of the roman republic