Until the End of the Sixteenth Century,the Ottoman Empire Was

Multiple Choice

Until the end of the sixteenth century,the Ottoman empire was on an almost constant war footing.Why?

A) The empire could be sustained only through raiding and conquest.
B) Ottoman wars to expand their empire kept powerful nobles on the frontiers, away from the capital where they might have threatened the sultan.
C) Christians launched a series of Crusades to recapture Constantinople, forcing the Ottomans to defend themselves.
D) The Mongols posed a persistent and very real threat on the Ottomans' eastern frontier.
E) all of these