Which of the Following Best Explains Why Tang China and the Abbasids

Which of the following best explains why Tang China and the Abbasids competed directly for dominance in central Asia, in contrast to the relationship of the Han and Roman empires?

A) Afro-Eurasia had become more closely integrated through trade, conversion, and political contacts, as both the Tang and Abbasids expanded into central Asian border regions.
B) During the Roman and Han empires, there had been no trade between east and west, so central Asia was not so important.
C) During the Roman and Han empires, central Asia was a power vacuum, permitting bandits to attack the trade routes and inhibiting interaction.
D) Powerful steppes tribes such as the Sasanians prevented the Han and Romans from expanding into central Asia, while they encouraged the Abbasids to expand toward the Tang borders.