a. Which variety was more common when there was more air pollution? Give a reason.
The dark-colored (melanic) variety was more common when air pollution increased because soot-covered trees made it easier for dark moths to camouflage and avoid predators.
b. Which variety is more common in England nowadays? Give a reason.
The light-colored variety is now more common because air pollution has decreased, making tree bark lighter. This provides better camouflage for light-colored moths and increases their chances of survival.