Primary producing countries experienced low and unstable prices in the 1920s because of

a. increased food production by food importers during the First World War and high levels of debt

b. increased food production by food importers during the First World War and decreased incomes in urban areas

c. the "dust bowl", which caused a fall in agricultural production, and rising incomes, which caused a shift in consumption from basic foodstuffs to more income elastic products.

d. increased agricultural productivity and rising incomes, which caused a shift in consumption from basic foodstuffs to more income elastic products.