Why Did the Huns Ultimately Direct Their Energies Westward and Not

Why did the Huns ultimately direct their energies westward and not toward the eastern half of the empire?

A) The western emperors invited the Huns in under the condition that they serve in the western army.
B) The Huns had adopted the Arian form of Christianity and sought to punish the western church for its persecution of heresy.
C) The Huns' move toward Constantinople was blocked by the Alps.
D) The eastern emperors paid the Huns to spare the eastern half of the empire.