Luther Burbank bred potatoes that combined a variety of useful traits. One of Burbank's breeding techniques was hybridization. Which cross of potato plants would have been most useful for producing a useful strain by hybridization?

a. a disease-resistant plant crossed with a plant that is not disease resistant
b. a plant that is not disease resistant crossed with a plant that produces a low volume of food
c. a disease-resistant plant crossed with a plant that produced a low volume of food
d. a disease-resistant plant crossed with a plant that produced a high volume of food