Natural Selection Has Favored the Dark Form of the Peppered

Natural selection has favored the dark form of the peppered moth in areas subject to severe air pollution,perhaps because on darkened trees moth-eating birds see them less easily.As pollution abated,the light forms were favored because

A) they were less visible after the lichens returned.
B) of migration from more polluted areas.
C) of mutation.
D) they were preyed on more.
E) of a bottleneck effect.