Valerie is training for a triathlon, a timed race that combines swimming, biking, and running. While Valerie usually practices swimming for two hours per day, she decides to continue for an additional hour because the pool is less crowded than usual today allowing her practice time to be more productive.

Which basic principle of individual choice does Valerie's plan illustrate that her husband's advice does not?

a. Resources are scarce.

b. All decisions involve opportunity costs.

c. Decisions are made at the margin.

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