Researchers studying blind cave fish that lack eyes found that crossing two pure-bred fish from the same cave gave only blind progeny. However, when they crosses two pure-bred cave fish from separate caves, all the progeny were able to see. From this result, they can conclude about the mutations which cause blindness in those cave fish in different caves are:

A. Located in separate genes
B. Display epistasis where one gene masks another
C. Located in the same gene
D. Located in two separate genes that must be right next to each other on the same chromosome
E. Form an allelic series in the same gene