Grazing livestock on public land in the we

题目：

Grazing livestock on public land in the western United States is not causing widespread environmental damage in the region, since if it were, the condition of that land would not be improving. However, only 14 percent of public land in the area today is considered to have inadequate vegetation cover and, therefore, to be in poor condition, while in the 1930’s, 36 percent had inadequate vegetation cover.
Which of the following, if true, most seriously weakens the argument above

A．In the western United States, private land is typically more lush than public land, and cattle that graze on private land can be fattened more quickly.

B．Since the 1930’s, recreational users of public land in the western United States have caused more environmental damage than have the cattle grazing there.

C．During the 1930’s, an unusually destructive drought prevailed throughout the region where most public lands in the western United States are located.

D．Ranchers who use public land in the western United States pay only a fraction of what is paid by those who lease comparable private land for grazing.

E．(E) The amount of land purchased by the United States government since the 1930’s is relatively insignificant.