Concept:The population clusters in these northern Nigerian cities were largely driven by economic activities linked to agriculture.
Explanation:In the 1960s, Kano, Kaduna, and Zaria were important centres for the production and trade of major cash crops such as groundnuts and cotton.
These crops attracted traders, labourers, and migrants, leading to dense population clusters in these cities.
Thus, the clustering is best explained by the presence of major cash crops, not mainly by culture, climate, or industrial employment.
Answer:C. the existence of major cash crops