To many environmentalists, the extinction of plants—accompanied by the increasing genetic uniformity of species of food crops—is the single most serious environmental problem. Something must be done to prevent the loss of wild food plants or no-longer-cultivated food plants. Otherwise, the lack of genetic diversity could allow a significant portion of a major crop to be destroyed overnight. In 1970, for example, southern leaf blight destroyed approximately 20 percent of the United States corn crop, leaving very few varieties of corn unaffected in the areas over which the disease had spread. Which of the following can be inferred from the passage above
A．Susceptibility to certain plant diseases is genetically determined.
B．Eighty percent of the corn grown in the United States is resistant to southern leaf blight.
C．The extinction of wild food plants can in almost every case be traced to destructive plant diseases.
D．Plant breeders focus on developing plants that are resistant to plant disease.
E．(E) Corn is the only food crop threatened by southern leaf blight.