Many factors have contributed to the great increase in desertification in recent decades. A. Growing human populations and the agricultural demands that come with such growth have upset the ecological balance in some areas and led to the spread of deserts. B. As periods of severe dryness have become more common, failures of a number of different crops have increased. C. Excessive numbers of cattle and the need for firewood for fuel have reduced grasses and trees, leaving the land unprotected and vulnerable. D. Extensive irrigation with poor drainage brings salt to the surface of the soil, a process that reduces water and air absorption. E. Animal dung enriches the soil by providing nutrients for plant growth. F. Grasses are generally the dominant type of natural vegetation in semiarid lands