Rajiv 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, Rajiv 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 Rajiv's plan illustrate that his wife's advice does not?
A. People usually exploit opportunities to make themselves better off.
B. Resources are scarce.
C. Many decisions are made on the margin.
D. All costs are opportunity costs.

In the given case, time is a scarce resource and limited. Rajiv has to practice for triathlon which comprises of 3 events. To win the triathlon, he needs to perform well collectively.

If he undertakes to swim more, the time available for other 2 activities need to be sacrificed i.e running and biking. Rajiv has planned to sacrifice biking marginally in the favor of swimming. The lesser an activity is sacrificed, the lower would be it's opportunity cost.

Whereas his wife's approach in short can be called "all or nothing" approach which relates to completely withdrawing from other activities and accepting one activity fully.

Rajiv is trying to make little changes instead of fully giving up on one activity and totally taking up another. This involves lesser cost in the form of opportunity cost, at the same time help him prepare for all three activities efficiently.