Scarcity, opportunity cost, and marginal analysis Susan is training for a biathlon, a winter racing sport that combines cross-country skiing and rifle shooting. Consider the following scenario: Because her ski training sessions are helping her quickly improve at skiing, Susan plans to reduce the time she spends training at the shooting range by an hour, and increase the time she spends skiing by an hour. However, her training partner says that she should pause all shooting practice and spend the entire 15 hours this week in the pool. Which basic principle of individual choice does Susan's plan illustrate that her training partner's advice does not?