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Answer:B) use of slave labor.Explanation:During the high middle ages most slaves in Europe were due to religious wars. E.g. if Christian crusaders won territory they would take Muslim slaves and when Muslims won territory they would also take Christian slaves.During the early middle ages slaves were used for agricultural activities but that almost ceased in the high middle ages. The Catholic church promoted the change from being a slave into being a feudal serf. By the late 10th century (end of early middle ages) most slaves were turned into serfs. Even though the life of a serf was very complicated and difficult, it was better than being a slave.But that didn't apply to religious prisoners until the end of the high middle ages when the Pope prohibited Muslim slaves (but that didn't stop it from happening)....