Final answer:
Desertification is caused by human activities such as overgrazing and deforestation, which lead to land degradation. Flooding generally does not cause desertification. The correct answer to the question is option 4, which includes both overgrazing and the cutting of trees for firewood. Option 4 is the correct answer.
Step-by-step explanation:
The process of desertification is mainly brought about by human activities that degrade the quality and productivity of the land, leading to the transformation of fertile areas into deserts. This includes practices such as overgrazing by livestock, which strips the soil of vegetation and makes it more prone to erosion.
Equally harmful is the cutting of trees for firewood or other purposes, known as deforestation, which removes tree cover that binds the soil and retains moisture. Both overgrazing and deforestation create conditions where the land can no longer support plant life, ultimately leading to desertification. Flooding is not typically a cause of desertification, but rather a natural event that can sometimes contribute to soil fertility.
Therefore, the options that cause desertification include overgrazing and cutting of trees for firewood. In considering the provided information and its consequences, the correct option in the final answer would be option 4) overgrazing and cutting of trees for firewood.