Paolo is training for a triathlon, a timed race that combines swimming, biking, and running. Consider the following sentence: Paolo has only 20 hours this week that he can devote to training. Each hour he spends swimming is an hour that he can't spend biking or running. Which basic principle of individual choice do these statements best illustrate?

a. Juanita can use time most efficiently by spending the same amounts of time on swimming, biking, and running.
b. People face trade-offs
c. People usually exploit opportunities to make themselves better off.
d. Juanita has an incentive to spend more time on swimming than on biking or running.

People usually have different options of a toons that can be taken with their various benefits. These different options available to an individual are called trade offs.

When a particular line of action is taken, the alternative action forgone is called the opportunity cost.

In economics the opportunity cost is also considered along with the actual cost of an activity.

In this scenario Paolo has trade offs of swimming, biking, or running.

However he has only 20 hours this week that he can devote to training. Each hour he spends swimming is an hour that he can't spend biking or running.

So biking and running are the opportunity costs he has to pay for swimming