The Irish Potato Famine of 1846-1847 and the U

The Irish potato famine of 1846-1847 and the U.S.epidemic of southern leaf blight of maize in 1970 resulted from:

A) increases in the mutation rate.
B) mistakes in genetic engineering.
C) the overuse of chemical sprays.
D) widespread cultivation of genetically uniform varieties that were susceptible to disease.
E) an unusual abundance of numerous types of plant pathogens.