The correct answer is: C. Gabriel’s demand curve for each activity.
Gabriel’s demand curve for each activity is the relationship between the price of each activity and the quantity of each activity that he demands. It can be derived by changing the price of each activity holding his income constant. When the price of one activity decreases, Gabriel will demand more of that activity and less of the other activity. This is because he can now afford to buy more of the activity that has become cheaper.
A supply curve is a graph that shows the relationship between the price of a good and the quantity of that good that producers are willing and able to supply. It is derived by holding the costs of production constant and changing the price of the good.
A net gain is the difference between the total benefits and the total costs of an activity. It can be calculated by adding up the benefits of the activity and subtracting the costs of the activity.
A marginal benefit is the additional benefit that a person receives from consuming one more unit of a good or service. It can be calculated by taking the difference between the total benefit of consuming two units of a good and the total benefit of consuming one unit of the good.