Which characteristics describe a tundra climate? A. extreme heat with heavy rain for several months B. four equal seasons with few extremes C. long summers, short winters, and little rain D. short summers, long winters, and permafrost

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?