Refer to the Figure Below, and Then Answer the Question

Multiple Choice

Refer to the figure below, and then answer the question that follows.

-Migration of the salamander Ensatina eschscholtzii separated an original population as they moved either west or east of the Central Valley of California.Eastern and western species then evolved.The Central Valley is an example of a/an:

A) temporal isolating mechanism.
B) mechanical isolating mechanism.
C) extrinsic isolating mechanism.
D) intrinsic isolating mechanism.