The Range of a Species of Vole Starts Contracting at Its

The range of a species of vole starts contracting at its edges. Slowly, the contraction continues. Eventually, the population can persist only in what used to be the center of its range. Based on this information, the most likely cause of the range contraction is

A) an invasive species.
B) climate change.
C) small population size effects.
D) background extinction.