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

Consider the following sentence: Because her pool sessions are helping her swim more quickly, Eleanor 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 Eleanor's plan illustrate that her husband'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.

which one is the answer?