How is Christopher Columbus indirectly responsible for the Atlantic slave trade?

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Which of the following led to a continuation of the treatment in the excerpt above? a)the brutality of the trans-atlantic african slave trade. b)spanish repression after violent native uprisings in new spain during the 16th century. c)european competition to quickly establish colonial holdings in the new world. d)the reliance of the spanish on native americans to provide labor under the encomienda system.

What’s the term used for a lawyer who argues for the convection for a defendant in a criminal case?