重温一下这道题。。。

which of the following most logically completes the argument?

when people engage in activities that help others, theirbrain release 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________

a. in the communities studied, women were much more likely to do regular volunteer work than men were, and women tend to live longer than men do

c. the feelings of well-being induced by endorphins can, at least at a time, mask the symptoms of various conditions and diseases, provided the symptoms are mild

答案是a。能有人再给清楚地解释一下为啥是么？