Why africa is very poor continent though it is very rich in natural resources

Although the African continent is blessed with gold, diamonds, oil, coltan, bauxite, uranium, iron ore and other valuable resources, its inhabitants have long numbered among the world's poorest. While a few sub-Saharan African nations are doing relatively well, most are mired in poverty.Africa is continent made up of 50+ countries, some of which are resource rich, others not so much, some particularly those in the north.

answer; /// i believe the correct answer is (b) disposition

b) to encourage free trade