a. a new mutation allowed moths to survive
c. the phenotype frequency changed
Originally, the vast majority of peppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens upon which they rested. However, due to widespread pollution during the Industrial Revolution in England, many of the lichens died out, and the trees which peppered moths rested on became blackened by soot, causing most of the light-coloured moths, or typica, to die off due to predation. At the same time, the dark-coloured, or melanic, moths, carbonaria, flourished because of their ability to hide on the darkened trees.
Because of this, there was a great rise in the frequency of alleles producing melanic phenotypes that correlated with the general blackening of the surrounding environment.
I’m also currently taking AP Biology. (: