Concept:The sparse population of the West African middle belt is best explained by historical human loss during the trans-Saharan slave trade, not by resource availability.
Explanation:In the past, the middle belt was a major target for slave raiders.
Many people were captured and taken away, which sharply reduced the population.
The constant danger also discouraged families from settling there permanently.
On the other hand, rich fertile soil and mineral resources are factors that normally attract people.
A concentration of industries would also encourage higher population density.
Therefore, these positive conditions cannot account for low population numbers.
The trans-Saharan slave trade remains the most direct historical cause of the region’s low population density.
Answer:A. trans-Saharan slave trade