Answer:
The basic principle is known as the opportunity cost
Explanation:
The opportunity cost is defined as something that you are not earning because you don't do a revenue activity.
In this case, Sharon doesn't receive $9 per each hour that she prefers to go to the swimming pool. Also, she is expending $4 additional to the opportunity cost each time that she goes to swim.
For example, if she goes to swim 2 hours a day instead of work them, we can conclude that she isn't earning $18 and lossing $4 additional for the fee entrance to the swimming pool.
Please, note that, are different the concepts of "let of earning" and "lossing". First one talks about money that you never have had and the second is about money that you had but now you don't.
