Concept:The dry season in Nigeria is caused by the dominance of a dry, dusty desert airmass.Explanation:Nigeria has two main airmasses: the tropical maritime airmass, which brings the rainy season, and the tropical continental airmass, which is hot and dry and brings the dry season. The Harmattan is the dry, dusty wind associated with the tropical continental airmass, but the airmass itself is the main cause of the dry season.Answer:B. Tropical continental airmass