Juanita is training for a triathlon, a timed race that combines swimming, biking, and running. Consider the following sentence: Juanita 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: 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.

The above choices that Juanita has all have opportunity costs. If she opts to swim, she forgoes the time that she could have spent biking or running. So the opportunity cost of an hour of swimming is an hour of biking or an hour of running, whichever is the highest-valued alternative she must sacrifice in order to spend an additional hour swimming.

There is however no more information to determine if Juanita has an incentive to spend more time on swimming than on biking or running or the other option.