What three empires in western africa flourished because of the trans-saharan trade of gold and salt?

Greg goes to a high school and observes the way students talk to each other. he makes notes on the words used, the atmosphere of the school and the way the students present themselves to each other. this is an example of a( research method.

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?

Black death spread because of what problem in medieval europe? which one is it i need the answer quick . a- overcrowded cities b-too many farm animals c-not enough qualified doctors d-all of the above