Use the map of Africa below to answer the following question: Physical map showing the continent of Africa. The area surrounding the Niger River is green and labeled Inland Delta. The Atlantic Ocean, Mediterranean Sea, and Indian Ocean are labeled. The Niger River and Inland Delta surrounding it are labeled. The Atlas Mountains are labeled along the northwestern coast and are shaded dark orange. The Sahara is labeled in the north-central part of the continent and is shaded orange, and the Sahel is labeled south of the Sahara and is shaded light green. A key is included that shows green as the lowest elevation, yellow as moderate elevation, orange as high elevation, and red as the highest elevation. Most of the western part of the continent is light to dark green with some areas shaded light orange. The north-central part of the continent is mostly light to dark orange, with some areas shaded light green. Most of the southern part of the continent is light orange to dark orange to light red, with the coastal areas shaded light to dark green.
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What geographical feature accounts for why camel caravans were an important innovation in trade in West Africa?

Niger River
Atlas Mountains
Sahara Desert
Mediterranean Sea