Explanation:

All the above statements are correct.

Some of the world's largest supplies of underground water exist beneath the Sahara Desert, supporting about 90 major oases there. ... This means that whoever controls an oasis also controls the trade along the route—making oases desirable to political, economic, and military leaders.

 The Arabs have retreated to Saharan cities like Cairo; at roughly 10 million people, Africa's largest. Trucks are replacing camels in the salt trade. Tuaregs are acting as guides to Western adventure tourists and oil and gas operations promise far greater riches than gold and ivory ever could.

The discovery of oil is constantly transforming the Sahara desert. The discovery of oil has been made in Algeria, Libya and Egypt. Other important minerals found in this area include iron, phosphorus, manganese and uranium.