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

Consider the following sentence: Because her pool sessions are helping her swim more quickly, Maria plans to reduce by one hour per week the time she spends training on the bike and increase by one hour the time she spends in the swimming pool; but, her husband says that she must stop doing any running and biking 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?

1. Resources are scarce.

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

3. All costs are opportunity costs.

4. Many decisions are made on the margin.