Is there one explorer you'd consider the most important to development of our country

Population parameters are difficult to calculate due to

Martha participates in a research study in order to receive extra credit for her psychology class. when she arrives in the research office she is given a can of what looks and tastes like beer, but which actually contains no alcohol. in the adjoining room martha's classmate karen is also participating in the study. karen receives a can of alcoholic beer. if the experiment was designed to test the effects of alcohol on concentration, then who is exposed to the independent variable?

Which environmental change allowed people to travel by land between asia and north america a. human apaption to arctic environments b. the introduction to horses to north america c. the formation of ice sheets on land d. a rise in ocean temperatures