Which of the following is an assumption on which the argument relies?
AIn the past 10,000 years, the only trees to have grown above today's timberline are the trees whose fossilized remains been found.
BNo trees grew 10,000 years ago at altitudes higher than the ones at which fossilized tree remains have been found.
CThe fossils are not of species of trees that were able to tolerate cooler growing temperatures than are the species that currently grow near the timberline.
DThe Rocky Mountains have not eroded significantly over the past 10,000 years.
EThe climate of the Rocky Mountains has never been significantly warmer than during the lifetime of the trees whose fossilized remains have been found.