Crystal is training for a triathlon, a timed race that combines swimming, biking, and running. Because her pool sessions are helping her swim more quickly, Crystal 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 this scenario illustrate?

A. Many decisions are made on the margin.

B. People usually exploit opportunities to make themselves better off.

C. All costs are opportunity costs.

D. Resources are scarce.