During the Abbasid dynasty, what event marked the permanent end of the caliphate?
 a)The Umayyads took over and established a different kind of government.
 b)Mongols entered the city of Baghdad and killed the caliph.
 c)The Fatimids took power and established a new capital in Kabul.
 d)The Abbasids themselves declared an end to the caliphate.