The variable is the group that was given caffine, because thats what the doctors manipulated. They possibly altered the natural memory of the participants by giving them caffine. A more specific variable could be how much caffine each participant is given and what time are they given it.
Answer:
Marginal analysis is helpful to individuals and businesses in balancing the costs and benefits of additional actions, like whether to produce more, consume more, and similar other decisions, thus determining whether the benefits will exceed costs and increase utility.
I see most of the statements are correct. Please see if the question asks for INCORRECT statement instead. If so, then the answer is In a Market economy, competition is controlled by the government.
If the question is really asking for all correct statements, please mention in comment and I will list them out.
