The Two Squirrel Species Shown in the Figure Below Arose

The two squirrel species shown in the figure below arose from the same ancestor. The Kaibab squirrel lives on the north rim of the Grand Canyon and Albert's squirrel lives on the south rim. Which is the most likely explanation for their speciation?

A) The population of ancestral squirrel occupied only one of the rims of the Grand Canyon. As the population grew, some members of the population crossed the Grand Canyon to form a new population. The populations had different phenotypes due to the founder effect.
B) The population of the ancestral squirrel was split by the formation of the Grand Canyon. Since the habitats were different on each rim, the populations evolved differently and became reproductively isolated.
C) When the Grand Canyon formed, competition for food and shelter became fierce. The two species arose through sympatric speciation as some individuals gathered food on the north rim rather than the south rim.
D) The ancestral squirrel was actually a hybrid of two squirrel species. When the populations were separated and could no longer mate, the characteristics of the two species became more obvious.