How does this document help you address the question: What caused the dust bowl?

The Dust Bowl was the crisis that took place on the southern Great Plains during the 1930s. It was a period of drought and severe wind erosion in the southern and central plains, which was followed by economic depression. The northern parts of the Prairie were not affected as badly. The devastation of agriculture prolonged the Great Depression. It also led to great emigration. Poor farming practices helped to make the situation more difficult.

During the 1920s, many farmers moved to the plain when new opportunities to increase production were seen. The grasslands were plowed up to grow grain. During years of abundant rainfall, enough grain could be grown, but a long and difficult drying period began in the early 1930s. However, agriculture was run as before, which resulted in the soil becoming increasingly nutrient poor and the top layer holding the soil in place disappeared. A record crop of wheat in 1931 led to a fall in food prices and the farmers found it difficult to cope with their subsistence.

according to my calculation, the answer is d.