Exposure to a large dose of something that causes bodily damage -- such as excessive heat, poison, or nuclear radiation -- is of course harmful to an organism. But, surprisingly, exposure to small doses of such stressors has been shown to extend life span in various species, including fruit flies, protozoans, worms, and rodents. Which one of the following, if true, most helps to explain the surprising phenomenon described above? ['In most of the species in which exposure to small doses of stressors increases longevity, the increase is so small that it is barely measurable.', 'Repeated exposure to a stressor is much more likely than a single exposure to cause permanent damage to an organism.', 'Exposure to a given dose of a poison or other stressor may cause more serious damage to some members of a species than to others.', "Exposure to small doses of stressors stimulates an organism's natural repair mechanisms to fix any damage caused by the stressors as well as some unrelated damage."]