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Answer:
C. the spread of disease.
Explanation:
Adequate water and sewage systems are characteristics of modern urban centers. They became part of standard urban planning and management only in the late 19th century and early 20th century. The lack of such systems means unsanitary conditions and high probabilities of disease spread. We can still see public health problems originated in inadequate sewage and water systems in many developing countries.
With the ability to forge iron tools, farming tools were more effective and provided a greater production and effectiveness. More food produced means that more people are able to be fed and consequently establish and work in the land. With a higher population and food cap, West Africa settlements were able to grow.
The answer is in the text.
