The Kaibab Squirrel Lives on the North Side of the Grand

The Kaibab squirrel lives on the north side of the Grand Canyon, and the Abert squirrel lives on the south side. Even though these two populations are only miles apart, their gene pools are kept isolated by

A) temporal isolation.
B) ecological isolation.
C) hybrid infertility.
D) geographic isolation.
E) mechanical incompatibility.