Scientists have found that inserting genes from a plant virus into the genes of a plant susceptible to that virus increases the plant s resistance to the virus. Because viral diseases account for a significant proportion of crop losses, such genetic alterations, even if carried out on only a modest scale, will significantly reduce crop losses. Each of the following, if true, strengthens the conclusion above EXCEPT:

A. In areas where two successive crops are raised per year, protecting the first crop from a virus will generally protect the second crop from that virus as well.
By repeatedly attacking plants that are genetically altered to become virus-resistant, the virus often becomes less lethal to plants that are not genetically altered in that way.
C. Plants that are genetically altered to become virus-resistant often pass virus-resistant genes on to their offspring.
D. Plants that arc made genetically resistant to one kind of virus often acquire resistance to related viral strains as well.
E. Plants that are made genetically resistant to one kind of virus are then more susceptible to infection by unrelated viruses.