Compared to Greece, why was Persia able to organize such a massive empire?

A. Its territory was connected by flat plains, unlike Greece's steep mountains and numerous islands.
B. Its mastery of sea travel allowed it to govern lands that lay thousands of miles from its capital.
C. Its many islands were relatively close together, allowing for easy communication and governance.
D. Its population was more culturally unified than Greece's population, which consisted of many different ethnic groups.