Over the past century, sagebrush has dramatically expanded its range over the Colorado plateau's grasslands. This development has been attributed to the overgrazing of the area's grasses by cattle and sheep. This example indicates that

A) sagebrush is the dominant competitor over grasses.
B) the relationship between sagebrush and grasses is mutualistic.
C) sagebrush could not exist anywhere in the absence of cattle and sheep.
D) the range of sagebrush is normally limited by competition with grasses.
E) grazing has no effect on the distribution of plants.