A Biologist Has Been Studying Two Populations of Trout in Montana

A biologist has been studying two populations of trout in Montana for the last 25 years, when the two populations were established from one common population. In the southern population, the fish now have a slightly different mouth shape. But the northern population hasn't changed from the original type. The new shape works well in catching the type of prey common in the habitat of the southern population. Which is the best explanation for the gene differences in the two populations?

A) The mutation only appeared in the southern population after it was exposed to the new prey.
B) The gene that produces the variation is probably still present in a small percentage of the northern population, but since there's no advantage to having it, individuals with the variation aren't selected.
C) The gene that produces the variation used to be present in a few individuals, but since it wasn't advantageous, individuals with that variation died out.
D) The gene that produces the variation used to be common in both populations, but since it wasn't advantageous in the northern population, the variation died out.