Why Did the Nutritional Quality of Native Diets Become Substantially

Why did the nutritional quality of native diets become substantially depressed in many parts of Africa?

A) Changes introduced by European intruders caused the spread of the tsetse fly,which damaged native crops.
B) The natives of the region practiced a scorched earth policy of defense against European intruders in which they burned their own fields.
C) European farmers resented the cheap imports that flowed into the continent after colonization,and colonial administrators were pressured to destroy millions of acres of crops owned by natives.
D) Cash-crop production supplanted the production of native foods.