填空题
[听力原文]11-15
The deserts of the world are not all covered with sand. Many of them have surfaces of (11) （）. They are not all flat, either. They often have (12) （）, There is a little rain in the desert, but it does not come regularly, and it does not fall often enough for most plants.
In the desert, the places where there is a supply of water are called oases. Sometimes the water comes from (13)（）, and sometimes it comes from rivers that flow through the desert.
The deserts of tile world are not uninhabited. But these people are not farmers. They have herds of (14) （）. These animals can live on (15) （） and do not need much water.