Read the passage carefully and choose the correct answer.

Approximately one-third of the Earth's land surface is desert, arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals. Deserts have been portrayed as fascinating environments of adventure and exploration. These arid regions are called deserts because they are dry. They may be hot. They may be cold. They may be regions of sand or vast areas of rocks with occasional plants.  

Deserts are natural laboratories in which to study the interactions of wind and sometimes water on the arid surfaces of planets. They contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Because deserts are dry, they are ideal places for human artifacts and fossils to be preserved. Deserts are also fragile environments. The misuse of these lands is a serious and growing problem in parts of our world.

There are many animals in the desert. Some are very large, like the kangaroo or the  gazelle. Both are big and have to travel long distances for water at a spring, or an oasis.  Another fairly large animal is the addax. The addax is a desert antelope. They live in the Sahara Desert. All addaxes are herbivores. There are less than 200 of them left because of hunting and tourists. Some animals crawl along. Examples of these are snakes and  lizards. Snakes rarely drink water; they get their moisture from other prey that they eat.  So do others, like the kangaroo rat. Lizards are commonly found in the desert. They stay  out of the sun and move as little as possible. There are also other animals in the desert.  The fennec lives in northern Africa in the Sahara Desert. They are a relative of the fox.  They eat mice, small birds, lizards, and insects. When necessary, they will eat fruit. One  more desert animal is the jerboa. The jerboa is a small, long-distance jumper that lives in the desert. They are free drinking animals and they eat plants, seeds, and bugs.  

The last paragraph expresses that ________.