What Was the Difference in the Military Organization of Byzantine

What was the difference in the military organization of Byzantine and western Roman empires?

A) The western Roman Empire depended on citizen soldiers until the 5th century.
B) The Byzantine Empire recruited men from the Middle East.
C) The Byzantine Empire recruited barbarians almost exclusively while the Roman Empire of the West depended on Islamic mercenaries.
D) The Byzantine Empire depended on the strength of Constantinople?s walls and did not recruit an army.