Answer:
A
Step-by-step explanation:
With the increase in pollution, the barks started getting darker. The grey squirrels were easily noticed by their predators against this dark background, so they were eaten.
Due to a random mutation, the black fur was acquired. Squirrels with black fur are now more able to camouflage and are less likely to be seen by predators. Those squirrels have a selective advantage, will be able to survive to mating age and pass on those successful alleles to the next generations. The number of grey squirrels (selective disadvantage) will decrease.
Those squirrels can be used to indicate the level of pollution a city is facing. The more pollution there is, the more the black squirrels will be. If efforts are being made to reduce the levels of pollution, the numbers of grey squirrels should increase and that of the black squirrels should decrease.