Answer:
D. The topsoil and grasses were blown away because of severe drought.
Step-by-step explanation:
It was a period of severe dust storms in the American and Canadian prairies during the 1930s. The phenomenon was caused by a severe drought and the failure to apply dryland farming methods to prevent wind erosion.
The farmers did not have enough understanding of the ecology of the plains and conducted an extensive deep plowing during the previous decade.
I hope this answer helps you.