Maria is training for a triathlon, a timed race that combines swimming, biking, and running.Consider the following sentence: Because her pool sessions are helping her swim more quickly, Maria plans to reduce by 1 hour per week the time she spends training on the bike and increase by 1 hour the time she spends in the swimming pool; however, her husband says that she should stop doing any biking and running and spend all 20 hours per week in the pool.Which basic principle of individual choice does Maria's plan illustrate that her husband's advice does not?A. People usually exploit opportunities to make themselves better off.B. Resources are scarce.C. Many decisions are made on the margin.D. All costs are opportunity costs.

In order to determine which actions Maria should undertake, she must first do a benefit cost analysis. The actions that generate more benefits than costs should be taken. If she want to maximize her benefits from her available resources (training time), she should chose the actions whose marginal benefits exceed their marginal costs.

Maria should spend as much time in the pool as possible, as long as it doesn´t hurt her total performance including her running and biking performance.