Why Did Rome Lose Its Status as the Roman Empire's

Why did Rome lose its status as the Roman Empire's most important city as a result of Diocletian's reforms?

A) Its population shrank so dramatically because of the exodus of governmental officials that it ranked only as the fifth-largest city in the empire.
B) The Vandals and the Visigoths both sacked the city, since Diocletian's reforms had taken funds away from the armed forces protecting the city.
C) Diocletian had the Christian officials of the city executed, destroying the Christian leadership of the entire empire.
D) Diocletian divided the empire into smaller units, thereby turning Italy into a section of the empire like any other.