The reasons for the Arabs' (Muslim) successful conquest of the Middle East and north Africa was most likely due to
(A) the promise of booty to be won.
(B) overpopulation in the Arabian peninsula.
(C) the desire to convert others to Islam.
(D) the weaknesses caused by their long wars of Islam's two main adversaries, Persia and the Byzantine Empire.
(E) the unity provided by their faith in Islam.
Answer: D