Assume Seven Different Strains of Fly Have Been Isolated,each Shows

Assume seven different strains of fly have been isolated,each shows a recessive white eye trait.Crosses are performed as follows (w) indicates white-eyed progeny, (R) indicates wild-type red eyes. $$\begin{array} { l l l l l l l l } & \mathrm { A } & \mathrm { B } & \mathrm { C } & \mathrm { D } & \mathrm { E } & \mathrm { F } & \mathrm { G } \\\mathrm { A } & \mathrm { w } & \mathrm { w } & \mathrm { R } & \mathrm { R } & \mathrm { w } & \mathrm { R } & \mathrm { R } \\\mathrm { B } & & \mathrm { w } & \mathrm { R } & \mathrm { R } & \mathrm { w } & \mathrm { R } & \mathrm { R } \\\mathrm { C } & & & \mathrm { w } & \mathrm { w } & \mathrm { R } & \mathrm { R } & \mathrm { R } \\\mathrm { D } & & & & \mathrm { w } & \mathrm { R } & \mathrm { R } & \mathrm { R } \\\mathrm { E } & & & & & \mathrm { w } & \mathrm { R } & \mathrm { R } \\\mathrm { F } & & & & & & \mathrm { w } & \mathrm { w } \\\mathrm { G } & & & & & & & \mathrm { w }\end{array}$$
Based on these crosses,how many different genes are present?

A) only one gene with several different alleles
B) 2
C) 3
D) 4
E) 7