Concept:Islam first spread into Nigeria mainly through economic contacts across the Sahara.Explanation:Between the 11th and 14th centuries, trans-Saharan trade brought North African merchants into the Kanem-Borno and Hausa regions.Through trade and commerce, these merchants introduced Islam to local rulers and communities.Jihads became a major factor later, especially in the 18th and 19th centuries, not in this earlier period.Answer:C. trade and commerce