Concept:A mono-economy depends mainly on one export product or natural resource for national revenue.Explanation:Nigeria earns most of its income from crude oil.Petroleum activities are carried out on a very large scale across the country.This heavy reliance on oil makes other sectors like agriculture and manufacturing less dominant.As a result, the Nigerian economy is described as a mono-economy because of the huge volume of petroleum activities.The other options do not explain the main cause of Nigeria’s mono-economic structure.Answer:A. huge volume of petroleum activities in the country.