Scarcity, opportunity cost, and marginal analysis Alex is training for a triathlon, a timed race that combines swimming, biking, and running. Consider the following sentence: Alex has only 20 hours per week that he can devote to training for his race. Which basic principle of individual decision making does this sentence best illustrate? People usually exploit opportunities to make themselves better off. Many decisions are made at the margin. Resources are scarce. All costs are opportunity costs.