The invisible hand is an economic term to describe the tendency of a market prices to lead individuals chasing their own interests into productive activities that encourage economic welfare of society too. With this explained, we can choose <em>C a term economists use to describe the self-regulating nature of the marketplace</em> as the correct answer.
Dr. Smith is conducting a longitudinal case where it is a case being done in a same manner over a long period of time. It could be seen above as Dr. Smith has instructed his representatives to go back every five years to conduct the same measurement in a course of a long period which is until they are sixty years old which could be classified as the longitudinal case.
