Scarcity, opportunity cost, and marginal analysis

Kate is training for a biathlon, a winter racing sport that combines cross-country skiing and rifle shooting.

Consider the following scenario: Kate has only 15 hours available to devote to her training this week. Each hour she spends skiing is one hour that cannot be spent working onher shooting.

Which basic principle of individual choice do these statements best illustrate?

Kate can use time most efficiently by spending the same amounts of time on skiing and shooting.

People usually exploit opportunities to make themselves better off.

People face trade-offs.

Kate has an incentive to spend more time on skiing than on shooting.