Corrupt government officials - this was the case
Increased disputes between the Muslims in the North and the Christians in the South - this was the case
Depletion of its oil
reserves - This is the correct answer. In fact the oil reserves of Nigeria started suffering depletion in 21 Century, but oil exports are still the basis of its export (almost 80%)
Political unrest as members of the various ethnic groups jostle
for political control - this was the case and included persecition of the Igbo people.
