[求助]向大侠致敬！请教一道题

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 incerases people's longevity. And a statistic in 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 comunities studied, women were much more likely to do regular volunteer work than men were, and women tend to live longer than men do.

B the number of young adults who do regular volunteer work is on the increase

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

D it is rare for a person to keep up a regular schedule of volunteer work throught his or her life.

E Some people find that keeping a commitment to do regular volunteer work becomes a source of stress in their lives.

这题选A, 我选的C,不大明白，请高人指点...感激不尽！！！