In mountainous regions, the timberline is the highest altitude at which trees grow. In the Rocky Mountains, the current timberline is at the altitude above which growing season temperatures remain cooler than 10 degrees centigrade. Fossilized remains of trees that grew 10, 000 years ago have been found 100 meters above the current Rocky Mountain timberline. Clearly, ttherefore, 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? ["In the past 10, 000 years, the only trees to have grown above today's timberline are the trees whose fossilized remains been found.", 'The climate of the Rocky Mountains has never been significantly warmer than during the lifetime of the trees whose fossilized remains have been found.', '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.', 'No trees grew 10, 000 years ago at altitudes higher than the ones at which fossilized tree remains have been found.']