People who do regular volunteer work tend to live longer, on average, than people who do not. It has been found that "doing good, " a category that certainly includes volunteer work, releases endorphins, the brain's natural opiates, which induce in people a feeling of well-being. Clearly, there is a connection: Regular releases of endorphins must in some way help to extend people's lives. Which of the following, if true, most seriously undermines the force of the evidence given as support for the hypothesis that endorphins promote longevity? ['There are many people who have done some volunteer work but who do not do such work regularly.', 'Although extremely high levels of endorphins could be harmful to health, such levels are never reached as a result of the natural release of endorphins.', 'People tend not to become involved in regular volunteer work unless they are healthy and energetic to begin with.', 'People who do regular volunteer work are only somewhat more likely than others to characterize the work they do for a living as "doing good. "']