The answer is a. lack of income.
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Impede means "stop." Not being rich does not stop the spread of culture. And by physical features, that means thing like "mountains" and "rivers", which can stop people from spreading out.
The Sahel is located in Africa. It is the region in Africa that
transitions from the Sahara to the Sudanian Savanna. It stretches across
in the region from the West of Africa to the East and includes parts of
Senegal, Mauritania, Mali, Burkina Faso, Algeria, Niger, Nigeria, Chad,
Sudan, South Sudan, Eritrea, CAR, Cameroon, and Ethiopia
Geologist study the lithosphere
Oceanographers study the hydrosphere
Meteorologist study the atmosphere
Astronomers study beyond the earth system and study the systems of other planets and heavenly bodies.
Environment scientists study all aspects of the Earth system, including the atmosphere, cryosphere, hydrosphere, geosphere, and biosphere.
25% of Africa are deserts!
I hope this helps;)
