After hearing strange noises from downstairs, you eventually realize that it was just your dog who had tipped over the garbage can. The subsequent slowing down of your heart rate is BEST explained by the activation of your system.

Chen's research team randomly assigned some newborns and their mothers either to a breastfeeding promotion group (where breastfeeding was encouraged, but not required) or to a normal pediatric care group (where breastfeeding was neither encouraged nor discouraged). which research method did they use?