Explanation: The Crusades were initiated in no small part by calls from the Byzantine emperor for help fighting the Seljuk Turks. Eliminate (A) because the Abbasids failed to conquer Constantinople. Eliminate (C) because the Avars and the Bulgars were not Muslims. Eliminate (D) because although the emperors of the Byzantine Empire in later years made strides in patching up religious differences between West and East, the majority of the Byzantine population remained staunchly Orthodox. The answer is (B).