Ana is training for a triathlon, a timed race that combines swimming, biking, and running. While Ana 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.

c. Decisions are made at the margin.

Explanation:

It is better to make a trade-off when someone compares the costs with that of  the benefits of doing something. The are called marginal decisions. Making the means comparing its costs as well as its benefits of an activity by doing it a little bit more versus doing a little bit less.

In the context, Ana is making decision at the margin when she decides to practice one hour extra and improve her swimming performance as the event of triathlon is approaching.

Thus, option (d) is correct.

answer///supply is elastic, and demand is inelastic;