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

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