语法填空

阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个适当的词或者使用括号中词语的正确形式填空。

    Over millions of years ago, there were deserts. Now about one fifth of the Earth's surface  ____ (be) deserts. The size of the world's deserts is always  ____(change). Deserts can be hot or cold. The Australian Desert is a hot desert, while the Gobi Desert in Mongolia is an example of a cold desert. Deserts are dry areas ____ less than 50 cm of rain falls each year. The Sahara Desert in Africa receives less than 1.5 cm a year. ____ rain doesn't fall very often in deserts, sometimes even for years, there can be heavy rain ____(sudden). Because they are dry, deserts have wide changes in temperature each day. When the sun  ____(go) up, it is very hot. At night, the heat  ____(receive) by the ground during the day  ____(quick) disappears(消失), and then it becomes very cold. Because  ____ the change, living in the desert is very difficult. Man can make deserts, but they can also stop them from getting larger and larger. Planting trees can prevent the sand from  ____(spread).