The Following Questions Refer to the Paragraph and Graphs Below

The following questions refer to the paragraph and graphs below.
In a hypothetical situation, the National Park Service, which administers Grand Canyon National Park in Arizona, builds a footbridge over the Colorado River at the bottom of the canyon. The footbridge permits interspersal of two closely related antelope squirrels. Previously, one type of squirrel had been restricted to the terrain south of the river, and the other type had been restricted to terrain on the north side of the river. Immediately before and ten years after the bridge's completion, ten antelope squirrels from both sides of the river were collected, blood samples were taken, and frequencies of alleles unique to the two types of antelope squirrels were determined (see graphs below) .


-The results depicted in the graphs above are best accounted for by which of the following facts?

A) The Colorado River has been an effective geographic barrier to these two types of squirrels for several million years.
B) There is more white fur on the tails of the northern squirrels than on the tails of the southern squirrels.
C) Both types of squirrel subsist largely on a diet of pine nuts and juniper seeds in this ecosystem.
D) Both types of squirrel share the same common ancestor.
E) Both types of squirrel are active during the day.