The correct answer is: b. occupies central and southern Africa, because the Sahara Desert is located in North Africa, this statement is not true.
It is true that it is almost as largest as the United States. The Sahara is 9.2 km2 and the United States is 9.8 km2. It is also true that the sand dunes can reach 590 ft high in the Sahara, almost 600 ft, and it is also true that the Sahara is at least 3 million years old, and recent studies says that it could be older.
