What began the development of the two distinct european civilizations of the western roman empire and the eastern byzantine empire?

The Reason why there was the Western Roman Empire and the Eastern Byzantine Empire was because the Empire was now too big to govern.  Emperor Diocletian divided into the East and West.  Later on, the Western Empire deteriorated and collapse due to frequent barbarian invasions. 

If the choices are:

A.  Leaders of the Catholic Church had a major disagreement in the seventh century.

B.  In the sixth century, trade flourished in the eastern regions but not in the west.

C.  Around 400 C.E., Germanic tribes took over much of the empire's western territories.

D.  In 395, the sons of Theodosius I each became emperor of part of the empire.

The correct answer is:  D