Before Muller's discovery that radiation induces mutation, scientists had to work on mutations that were found solely by phenotype differences in natural populations. Which of the features of Drosophila made it a fortuitous choice for Morgan and his colleagues?

A) having a long life cycle
B) especially high rate of mutation
C) large number of easily visible phenotypes
D) well-known biochemical pathways
E) the number of genes with only two alleles