1. Which statement is true? a. Congress may override the Russian president's veto.
b. The Russian president can create laws independently.
c. The Russian prime minister can make laws independently.
d. The Russian prime minister has the right to veto any law.

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?