The sweet potato is a common crop plant found in most warm areas of the world. The plant rarely flowers and more rarely makes seeds. However, the roots and potato vines spread rapidly, and new plants are easily grown from the roots and vines of old plants. Some scientists would like to grow new types of sweet potatoes that parasites will not attack. Which of these processes is most difficult in developing new types of sweet potatoes?

A. planting root stocks
B. cutting vines
C. removing parasites
D. selective breeding