Which one of the following statements on desertification is true? (1) The occurrence of desertification is limited to deserts such as the Sahara and the Namib. (2) Only the arid and semi-arid parts of the world are affected by desertification. (3) Desertification is the result of the interaction between certain land use practices and climatic conditions. (4) Desertification is the physical expansion of deserts such as the Sahara.