In mountainous regions, the timberline is the highest altitude at whic

Correct answer: C

A: Incorrect. The trees whose fossilized remains have been found need not be the only trees that had grown above today's timberline, since their existence alone is sufficient to advance the argument that the climate has cooled.
B: Incorrect. Similarly to (A), it is not material whether some trees grew at altitudes higher than the ones whose fossilized remains have been found. The existence of their fossilized remains alone suggests that the climate has cooled.
C: Correct. If the species of trees growing near the timberline 10,000 years ago could tolerate cooler temperatures, then the conclusion follows that they could have grown at higher altitudes at a climate temperature equal to the one today. In other words, a change in species, not a change in temperature, would have caused the reduction in the height of the timberline.
D: Incorrect. (D) is irrelevant: Accepting that the Rocky Mountains have eroded over the past 10,000 years, such erosion could have caused a cooling of the climate in the Rocky Mountains. Erosion of the Rocky Mountains is endogenous to any change in the climate.
E: Incorrect. (E) is irrelevant. Whether the Rocky Mountains' climate has ever been warmer is not material in order to determine that the presence of fossilized remains of trees that had grown above the current timberline is evidence that the climate has since cooled (as today's timberline is lower).

Conclusion: The climate of the Rocky Mountains is cooler now than it was 10,000 years ago.

Which of the following is an assumption on which the argument relies?

A. In the past 10,000 years, the only trees to have grown above today's timberline are the trees whose fossilized remains been found. -> Still fossils are found, so temperature is cooled now. Incorrect.
B. No trees grew 10,000 years ago at altitudes higher than the ones at which fossilized tree remains have been found. -> May be not, but we have fossils found, which means temperature is cooled. Incorrect.
C. The 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. -> So, fossils found are different type of trees. Now, the temp is not the reason, trees are different. Let's keep it.
D. The Rocky Mountains have not eroded significantly over the past 10,000 years. -> erosion and temperature can't be assumed to have a correlation. Incorrect.
E. The climate of the Rocky Mountains has never been significantly warmer than during the lifetime of the trees whose fossilized remains have been found. -> But, we are talking about tree above timberline. Incorrect.

So, I think C.

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