The article states: according to researchers, doing good increases the production of serotonin, the chemical in our bodies that can heal wounds and contributes to happiness. the same goes for endorphins, the brain's natural painkiller. in addition, studies have shown that people who act kindly and selflessly live longer, have more energy, and experience less stress, depression, and anxiety. why did the author include this passage?
a: to point out the fact that doing good can lower the levels of harmful chemicals such as serotonin
b: to describe ways that high levels of stress can cause a person to become more unhappy
c: to suggest that the world is a better place when people have more energy d: to show that being kind to others can increase a person's overall health and well-being