Which of the following most logically completes the argument?

When people engage in activities that help others ,their brain releases endorphins, the brain's natural opiates, which induce in people a feeling of well-being. It has been suggested that regular release of endorphins increases people's longevity. And a statistic on adults who regularly engage in volunteer work helping others shows that they live longer, on average, than adults who do not volunteer. However, that statistic would be what we would expect even if volunteering does not boost longevity, because____________.

【选项】the number of young adults who do regular volunteer work is on the increase