107. Read the following passage, then answer the question below.

In the 1930s, a series of dust storms swept across the southern Great Plains of the United States, turning midday into darkness and burying farms in drifts of fine soil. Contemporary observers blamed a severe drought, and drought was indeed a cause. But the disaster was also the product of decisions that had remade the landscape during the previous two decades.

After World War I, high wheat prices and the arrival of gasoline-powered tractors encouraged farmers to plow millions of hectares of native prairie grass. The grass, with its deep and dense roots, had held the soil in place through earlier droughts. Once removed, the land was left exposed. Farmers often planted wheat year after year without rotating crops or leaving fields fallow, and when prices collapsed in the Depression, many abandoned their land unharvested, leaving bare soil to the wind.

The federal government's response became a model for later environmental policy. The Soil Conservation Service, created in 1935, taught farmers techniques such as contour plowing, which follows the slope of a hill to slow runoff, and planting rows of trees as windbreaks. Farmers who adopted the methods received payments, and the agency employed demonstrators to persuade skeptical neighbors. Rain eventually returned in 1939, but historians credit the conservation measures, and not the weather alone, for the slow recovery.

The Dust Bowl is sometimes cited as a warning about relying on technology without understanding local conditions. Yet some economists caution against drawing a simple moral. Farmers were responding rationally to the incentives of the time, they argue, and the more important lesson may be about how markets and policies can reward practices that are harmful in the long run.

Which of the following is NOT mentioned as a contributor to the Dust Bowl?