Scarcity, opportunity cost, and marginal analysis

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

Consider the following sentence: Because his pool sessions are helping him swim more quickly, Eric plans to reduce by 1 hour per week the time he spends training on the bike and increase by 1 hour the time he spends in the swimming pool; however, his wife says that he should stop doing any biking and running and spend all 20 hours per week in the pool.

Which basic principle of individual choice does Eric's plan illustrate that his wife's advice does not?

A. Many decisions are made on the margin.

B. All costs are opportunity costs.

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

D. Resources are scarce.

(detailed explanation)