What Was the Nature of Slavery Within the Abbasid Social

What was the nature of slavery within the Abbasid social system?

A) According to the Qur?an, slavery could not exist in Islam, and the Abbasid freed all former slaves.
B) Slavery was limited to the non-Arab converts to Islam, and died out during the period of the Abbasid empire.
C) Because most unskilled labor was left to the unfree, slaves could be found in both the towns and countryside of the Abbasid empire.
D) Slavery was known in Abbasid cities, but was virtually unknown in the countryside where most labor was performed by a free peasantry.