The Dust Bowl of the 1930s was primarily caused by C. Poor farming practices, and D. Overpopulation in the Great Plains. The combination of excessive plowing of native grasses, which held the soil in place, and prolonged drought due to lack of rainfall (B. Lack of rainfall over consecutive years) led to widespread soil erosion and dust storms in the region. The Dust Bowl was not caused by above-average numbers of tornado events in the 1990s (A. Above-average numbers of tornado events in the 1990s), as the Dust Bowl occurred in the 1930s.