The correct explanation for some of the variation in gross domestic products (GDPs) shown on the map is:
A. Nigeria controls large oil fields.
This option accurately reflects how natural resources, particularly oil, can significantly contribute to a country's GDP. Nigeria, being one of the largest oil producers in Africa, has a GDP that is heavily influenced by its oil wealth. The other options relate to factors that may influence GDP but do not explain the variation as directly or strongly as Nigeria’s control of oil fields.