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According to paragraph 4, why is the human preservation of Douglas fir sapling trees a threat to the ponderosa pine forest? A. The presence of many sapling trees makes it more difficult for firefighters to reach the source of a forest fire. B. Douglas fir saplings are expensive to maintain, leaving little government money for forest-fire suppression. C. Saplings compete for space with the larger and more valuable fire-resistant trees. D. Dense areas of tall sapling trees can spread fire to the crowns of larger, fire-resistant trees.