The Sahel forms the border between the southern Sahara and the northern Sudan, a more fertile region below the desert. The Sahel is a band of somewhat dry savanna that runs for 2400 miles from coast to coast all along the southern edge of the desert. A savanna is a grassy plain with coarse grasses and only a few scattered trees. Since the Sahel receives more rainfall than the Sahara during the annual monsoon rains, people have been able to settle in this region and form empires for thousands of years. These empires, called the Sahelian Kingdoms, became rich and powerful by controlling the trade routes that crossed the Sahara. The control of these routes was invaluable because they connected the very limited number of oases in the vast desert. An oasis is an isolated fertile area in a desert that usually has a spring, well, or other source of water. The people of the Sahelian Kingdoms had a great deal of help conquering the sands of the great Sahara.
Why does the author include so many definitions in this passage?
A) He is making up words to describe an imaginary setting.
B) He is creating new vocabulary terms to describe desert life.
C) He wants readers to understand the difficult words he is using.
D) He wants readers to learn important words from other languages.