North American eastern white cedars grow both on cliff faces and in forests. Cedars growing on exposed cliff faces receive very few nutrients, and rarely grow bigger than one-tenth the height of cedars growing in forests, where they benefit from moisture and good soil. Yet few eastern white cedars found in forests are as old as four hundred years, while many on cliff faces are more than five hundred years old. Which one of the following, if true, most helps to explain the difference in the ages of the cedars on cliff faces and those in forests? ['Trees that are older than a few hundred years start to lose the protective outer layer of their bark.', 'The conditions on cliff faces are similar to those in most other places where there are few tall trees.', 'In areas where eastern white cedars grow, forest fires are relatively frequent, but fires cannot reach cliff faces.', 'Eastern white cedar wood is too soft to be used for firewood or modern buildings, but it is occasionally used to make furniture.']