Poornima 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, Poornima 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.

For one to take a particular action, one has to consider the benefits and costs that are involved and then see if the benefits outweigh the costs before the final decision is made.

Since her pool sessions are helping her swim more quickly, Poornima 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.

This means the extra time allocated to swimming is beneficial and the basic principle of individual choice that Maria illustrate is that many decisions are made on the margin.