Acause of the first crusade was the a. conquest of constantinople by the ottoman turks. b. invasion of spain by the muslim arabs. c. attack by the muslims on rome. d. conquest of asia minor by the seljuk turks.

 The first crusade is the attempt to recapture the Holy lands from the assaults of the Muslim invaders. There are three reasons in this movement naming: 1) to give freedom to Jerusalem and the Church of the Holy Sepulchre; 2) to defend the suffering Christians of the East and (3) to marshal the energy of the feudal lords and knights into a unified warfare. Among these reasons, the related ones is the invasion of Muslim Arabs in Europe. Answer is B.